Simon Joinson has the following conclusion on the JPG photo output quality from both cameras:

Not surprisingly given the family ties (and the fact it's the same lens), the output is very, very similar, with color, contrast and saturation almost identical. Look a bit closer and you can see – even in a JPEG – that there are some differences. The most obvious is the EP-1's visibly better pixel-level sharpness (thanks to a lighter anti-alias / low pass filter), though we also noticed that the highlight roll-off is a little harsher than the E-620 (something confirmed in our dynamic range testing) – it's better at ISO 200, but still not as good as the E-620. Overall though, it's great to see that Olympus hasn't just delivered on its promise to match SLR quality – it's actually surpassed it.

The lens used on both cameras was the Olympus 50mm f/2.0 Telephoto Macro ED.

Olympus E-620 Digital Camera Review at About.com – Kyle Schurman on the E620's image quality:

At 12.3 megapixels, the Olympus E-620 has plenty of resolution for large prints. Photo quality is very good most of the time, although you may have to adjust some of the manual settings in low-light situations, and shooting at high ISO settings results in very grainy photos.

Rating score for the camera was 4 / 5 stars.

Advantage: E-620.

The articulated LCD display on the E-620 lets you hold and shoot the camera at odd angles and orientations, say, to shoot a self portrait, or to hold the camera low to the ground, at waist level, or high up overhead.

Be aware that you'd need to engage Live view on the E-620 to take advantage of this feature. In Live View mode, the E-620 will use contrast detection AF (autofocus), which will be slower than the equivalent AF performance on the E-P1.

Read more on Olympus E-P1 vs Olympus E-620.

Olympus E-P1 vs Olympus E-620: High ISO Image Noise Performance – Brian Mosley:

The E-P1 has better noise control than the E-620, and is easier to use with fast (and often cheap) legacy lenses.

Advantage: E-P1.

Read more on Olympus E-P1 vs Olympus E-620.

Olympus E-P1 vs Olympus E-620: bbrault's Reasons – Buried in the middle of a long discussion thread on DPReview is this gem of a feedback by bbrault on why the E-620 was chosen over the E-P1.

I've reproduced the post below, with minor edits for clarity …

/*** Start of post ***/

I was also seriously considering the E-P1 but finally opted for the E-620.

I ordered it last week after a brief hands-on with both E-P1 and E-620.

Here is why:

1. Focusing Speed and Moving Subject Isolation. (Human Size Subjects)

I emphasize human subjects because whenever you bring up DOF control, there is always someone ready to demonstrate great DOF control and subject isolation for very small subjects (flowers, insects, birds, etc…) something very much easier to do than on human size subjects (specially for the 4/3 format).

Actually, have you ever seen a picture of a very large subject (building, large truck, etc…) with blurred background? And the other way around, have you ever seen a picture of an insect without a blurred background?

I like the ability to produce nice (human) portraits with blurred background.

This imposes a challenge on any AF system, especially if the subject is not still and moving around such as kids playing in the backyard. (No I am not talking sports).

Actually the narrower the DOF, the more precise the AF has to be, imposing an even greater challenge to the AF system even for relatively slow moving subjects.

While the E-P1 CAF (contrast detection auto focus) is relatively fast with its native m4/3 lenses, (slower CAF than G1 or GH1 but relatively fast); I found 4 problems: None of the available native m4/3 lenses are fast enough for portraits; CAF is still very slow compared to dedicated PDAF (SLR); E-P1 CAF is even slower with the 4/3 adaptor and most ZD (Zuiko Digital) lenses, specially with the ZD 50mm F2 and finally, there are no fast primes in sight for the m4/3 lens line-up.

2. No EVF and No Articulated Screen

I also like shooting live view whenever possible but only with an articulated screen.

The E-P1 claims almost 180 degree viewing but live view is at its best with an articulated screen, once you tried it you will not want to go back to a fixed view. The E620 provides this.

3. External Flash

I do quite a bit of shooting indoors and almost always with a bounce flash, never direct. (I hate direct flash except for outdoor mild fill-in).

Forget about the cute little E-P1 flash (Olympus FL-14) for bounce, it does not swivel and even if it could, it does not have the power.

Even the FL-36R has border line amount of power for bouncing.

In reality, the E-P1 has very little appeal except for its size.

Mind you its size is a major appeal and in that respect it is an exceptional camera.

Once you start mounting decent 4/3 glass and a FL-36R, you might wish you had the E620 body to hold it together.

Hope this helps.

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Read more on Olympus E-P1 vs Olympus E-620.

Advantage: E-P1.

Refer to DPReview's comparison.

Weight (inc. batteries)
E-620: 515 g (18.2 oz)
E-P1: 355 g (12.5 oz)

Dimensions
E-620: 130 x 94 x 60 mm (5.1 x 3.7 x 2.4 in)
E-P1: 121 x 70 x 36 mm (4.8 x 2.8 x 1.4 in)

Simply put, the E-P1 is smaller and lighter than the E-620.

Read more on Olympus E-P1 vs Olympus E-620.

Advantage: E-620.

The E-620 has a phase detection autofocus system, while the E-P1 has a contrast detection AF.

This results in the E-620 having a more responsive AF performance that's suited for quicker focus acquisition and lock.

Coupled with the availability of an optical viewfinder on the E-620 DSLR (none on the E-P1), this makes the E-620 the preferred equipment for such photography. Wikipedia states:

DSLRs are often preferred by professional still photographers because they allow an accurate preview of framing close to the moment of exposure …

Read more on the differences between phase and contrast detection AF, and on Olympus E-P1 vs Olympus E-620.

Advantage: E-620.

The E-P1 does not have an internal / pop-up flash.

Read more on Olympus E-P1 vs Olympus E-620.

Olympus E-P1 vs Olympus E-620 – Midway through this E-P1 review at Technocrati is the following comparative evaluation:

Frankly, the E-P1 produced even better JPEG quality than the E-620, thanks to the newer processor and improved algorithms.

Olympus E-620 Review at Digital Photography Review – The review awards high scores and a "HIGHLY RECOMMENDED" rating to the camera, and concludes that the E-620 is the best Four Thirds DSLR yet.

Canon T1i / 500D vs Nikon D5000 vs Olympus E-620 vs Panasonic G1 Noise Comparison at DPReview – The E-620 and Lumix DMC-G1 don't perform as well as the bigger-sensor cameras here, namely, the D5000 and EOS Rebel T1i / 500D / Kiss X3.

Olympus E-620 vs Panasonic G1 RAW Image Quality Comparison at DPReview – The Panasonic Lumix DMC-G1 wins on detail captured.

(via Olympus E-620 Review at Digital Photography Review)

Canon T1i / 500D vs Olympus E-620 RAW Image Quality Comparison at DPReview – The EOS Rebel T1i / 500D / Kiss X3 wins on detail captured.

(via Olympus E-620 Review at Digital Photography Review)

Nikon D5000 vs Olympus E-620 RAW Image Quality Comparison at DPReview – The D5000 wins on detail captured.

(via Olympus E-620 Review at Digital Photography Review)

Olympus E-620 dSLR /14-42mm Hands on Review and Sample Photos at Steve's Digicams – Steve issues the following verdict:

Olympus continues to offer high-quality features and abilities in their E-series models. While the E-620 is what we consider an advanced amateur model, it sports an entry-level price tag of about US$699 or less for the body/14-14mm Zuiko lens outfit. With excellent image quality both indoors and out, pleasing High ISO capabilities, and loads of easy to use features, we have no problem giving the Olympus E-620 dSLR a high recommendation.

There's also a gallery of full-resolution sample images with a nice variety of shots (taken with the Zuiko 14-42mm lens) for you to assess the image quality of the Evolt E620 with.

Canon EOS T1i / 500D vs Olympus E-620 vs Nikon D5000: Review, Sample Photos and Video Clips – This article at quesabesde.com is written in Spanish, so you might prefer to read Google's English translation of it.

The respective kit lens for each camera was used for the test: Canon EF-S 18-55mm IS VR on the T1i / 500D, Nikkor 18-55mm AF-S VR on the D5000, and Zuiko 14-42mm on the E620.

Tons of full-resolution images await your analysis.