Wesley feels that the Di866 is "… indeed a worthy (and cheaper) alternative to the original flash."

The two sample images in that forum post are not visible unless you're logged in, so see those pictures here.

Quantum Qflash / FreeXwire setup vs Nikon Speedlights / Flash Units – JPP801 shares some sample wedding photos, and says:

Not sure why you would ever not want to use them if you own them (Quantums) or have the chance to. On a wedding. the possibilities they open up are endless and in my opinion, not mimicable with all of this small strobe stuff. The ability to use real "bare bulb" alone, puts them on another level to Speedlights.

I had been shooting weddings, (lots of them), for years with SB80 / SB800's, (some people would say, fairly well) and never got anywhere near the quality of light etc, that I am now getting with Qflash and FreeXwire.

JPP801 shows more wedding image examples, this time, using the Quantum flash system for fill flash in the bright outdoors, and adds:

I think perhaps a lot of the problem here is expecting the speedlights to be able to do more than they are capable of or were designed for.

Invariably, they are not capable of putting enough power out or if they can struggle up to it, cant put it out often enough and/or frequently enough. That is even before you consider the shortcomings of the quality of light they produce.

Given the choice now, If at all possible, I will always choose to light with something like a Qflash, Lumedyne etc, over a speedlight. There are tons of reasons for this but as always, the images produced always tell the best story and I never have to worry about frying a flash or having enough output.

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Speedlights are ok if not alot of power is required and they are predominately used for fill where the main lightsource (low daylight, etc) can provide attractive modeling.

The Qflash setup that JPP801 uses is the Quantum Instruments QFX5DR TTL X5D Qflash Digital Flash System.

Nikon SB-900 vs SB-800: Output Power and Recycle Times – WNPmedia:

It's not the output power they improved but the recycle time. If you use the fifth battery with the SB800, it will still not re-cycle as fast as the 900. They both overheat it's just the 900 makes it more obvious. When you kill the temp monitor on the 900 it has fractionally more output – probably the wider lamp – but it fires faster at full power… And then dies faster! It also costs more to repair.

Nikon Speedlight SB-900 vs SB-800 – Joe Federer:

Turn off the Thermal Shutdown feature and you basically have a SB800 with a better interface and some better 'zooming' capabilities.

For me, that is worth it for the 'on-camera' one — not for the remote ones. So my on camera (and backup) flashes are 900's whereas my remote ones are 800's. I notice very little difference in power, if any, between them.

Nikon SB-900 Overheating Warning? – Paul Grupp:

My wedding photography partner and I have two SB-900s. While I agree that the sky is NOT falling, the fact is, the situation is less than ideal. When we're shooting in bounce mode where the distances are great enough to require full-power pops, the flash does get pretty annoying, flashing a warning on the LCD, and playing that irritating 3-tone warning.

My GUESS is that the thing is not going to melt down any faster than other flashes, but let's face it — the default setting of the warning circuit combined with the other warnings does not inspire confidence in those of us who worry about such things.

For Nikon to be taken seriously in the event flash world, they're going to need to address this issue in the relatively near future.

IMHO, the SB900 is a step forward compared to any other flash on the market — except in this one area.

Paul Grupp adds that the SB900 user manual does say that the protection function can be turned off:

The SB900 ships with a VERY conservative shut-down function enabled — the default setting is "uber-protection ON."

Turn off this function, and the flash will seem less like your enemy. With the protection on, the flash is unusable in any professional event capacity.

I agree that the achilles heel of the SB900 is its approach to thermal issues and self-protection, but if you turn off the protection circuit, it at least becomes usable, if not a complete success in a fast-pop environment. (It'll still flash a warning on the LCD and play an extremely annoying three-tone warning when it thinks it's too hot).

Put me down as a huge fan of the "new Nikon's" products like the D3X, D3, D700, 12-24/2.8, 24-70. These are the hits.

The SB-900 would have been a hit, IMHO, if it wasn't for the over-conservative thermal engineering.

The newish AF-S 50/1.4 G is in the same category — would have been a hit, but for it's outrageously sluggish AF performance.

Joe Federer confirms that he turned off Thermal Shutdown on his own SB900:

First thing I did when I got mine.

Mine's never ceased to fire.

(edit: I should mention I generally fire over 1000 flashes per reception – around 200-300 an hour).

Joe also gives the following tip:

You can turn off the beep, too, [...]