Nikon Speedlight SB-900 - my comments on using it.

It is not an official or systematic review, but rather several loose comments I have after using this flashgun in several different situations.I used it for indoor portraits, architecture/interior photography, outdoor portraits and landscape, as well as some night photography.

I used two other flashes for a while;  Nikon SB-600 which I love, and a Chinese flash, amazingly inexpensive, rather powerful and good - if you don't mind working with only basic settings - Yonghuo Speedlight YN460-II.
The Yonghuo has lots of bang for its money (just about $50 on ebay) - amazing value, works great in manual mode, and is perfect for general use, or as a backup flash.
Nikon SB-600 for quite a while worked for me as primary flash, and did it really well. Precision, power, endurance. I love it.   But, since the news about new super-flash Nikon SB-900 showed on line a while ago, i knew I want to use that one to.
So, some months ago I purchased it, and after quick tests decided to use it as my primary flash. It has remarkable power, much bigger than anything else around in its class. The power of SB-900 can be compared with power of some of bigger and much heavier studio type strobes, and is great outdoor, as it can overpower sunlight in some situations. Actually - its power was the major factor behind me ordering it.
So, I accepted that the power comes at cost, and at size - as it is bigger and heavier than SB-600.
Someone called it a "speedlight flash on steroids", and after a few tests I agreed with that.

It has such bang that it works great with biggest umbrellas, soft boxes, and other diffusers and modifiers, giving substantial light even after reflecting through several layers.

It also eats up batteries, much more than slimmer SB-600. I learned to always have another set or two of freshly recharged AA batteries. (by the way, I use remarkable rechargeable batteries I found in Hong Kong on ebay. They are orange-coulored CFL High Capacity AA batteries,  with amazing 3800mAH of energy, more than 4x of typical alkaline battery! So cheap, and they go really long way.)

High energy use is unfortunately not the only issue with my new SB-900 flashgun. There is a problem which I find really annoying and difficult to deal with; overheating!
It is all fine if you use your flash only sparingly. But for portrait works, or for HDR imaging, I need my flash to act often and sometimes in series of several flashes per minute. With that, I don't need to work long to hear the sound signal announcing overheating. After playing a lovely, happy melody - flash switches off for several minutes to cool down, and is simply unusable. What? Do I need to say "wait" to all people i am working with, so the flash can cool down?
It created several very silly situations for me, and also some situations where i was unable to complete my job in scheduled time.  Rather bad. Awfully bad, to be frank.
It has an override setting to disable overheating warning, but since it was factory programmed to switch off, can I risk ignoring it and frying the $500 unit? Not very tempting.

There is another option - to turn its power down, as when it is set to 1/4 or less, it does not overheat so quickly.
But the power was what I bought it for. So, if I need to reduce its power, I can rather work with smaller SB-600 which is lighter, much more gentle on batteries and easier to travel with. You can also try high zooming of flash beam to 180 or 200, as it tends to run a bit longer without overheating - but it may not be what you need on your set, so it can't be considered a working solution.

My final opinion after using it for 1/2 year?
It is a good flash which can SOMETIMES give you extra power when you REALLY need it.
But if you need consistent high power, use strobes in studio situations.
and, outdoor, on locations? turn your ISO up, and work with maximum power of SB-600 or other flashes you can get - or with SB-900 switched to 1/4 power or less.
and use higher power only occasionally, for selected shots, and only when you really need it.

I find this sole overheating issue very disappointing, and not worth the Nikon name. Perhaps Nikon's engineers should think of some cooling vents, or adding some sort of heat-sink? I don't know, but I am certain that solving this issue would greatly improve usability of this pricey unit. as it is for now - I have mixed feelings, and am not thrilled.  And, when I go for location work far away, traveling light - having pricey SB-900 at home, i travel with my old SB-600 which I find more reliable and trustworthy...