I finally took the plunge about a month or two ago. I'll admit - I was originally a skeptic (I'm not a gear queer, and I tend to be a cheap SOB), but I'm a NightVisionPen convert.
I've been using it regularly for the past few weeks, and since I fly a lot of student NVG events, it has gotten plenty of use. My experience has been nothing but positive. The light is plenty bright enough to illuminate your writing surface, but not so bright that it illuminates everything in the cockpit when you use it. In fact, the light emitted is similar to tearing a slit in a chemlight package. The light is much friendlier than that emitted by the Sidewinder, and it's nice not being "that guy" who leaves his liplight on all the time.
The thing I like best is that I no longer have to aim my liplight (and, by extension, my head) at my kneeboard in order to illuminate my writing surface - I can keep my head up, and peek down under the goggles to make my notes. It's too early to tell if this minimized 'heads-down' time makes for an increased safety margin, but I suspect moving only my eyes, vice my whole head, has got to increase my SA. I'm still trying to un-learn my habit of using my liplight, though - I've been using it for several years, so it'll take a bit for me to remember that I don't need it to write anymore.
Stinky's amateur hand model post from 29 Jul 10 (post #60 in this thread) pretty much sums up the specs, so I won't re-hash those. I've seen a lot of posts from people who want a flightsuit friendly version. I'm not one of them, as I don't use a pen from my flight suit pocket while in the aircraft. I'm wary of FOD (I've had more than a few crewmembers drop their pens, only to have them roll around the cyclic/pedals), so I insist that my pen be dummy-corded to my kneeboard. I use it in conjunction with the supplied lanyard and the 9G kneeboard, and have had zero issues thus far. The supplied lanyard works well, and the pen fits easily into the 9G's side pocket. I haven't had any problems with the light being left on inadvertantly - it takes positive pressure to activate the light switch.
Overall, the NVP is a great product for night owls. If you don't fly goggles all that often, or if your cockpit is lit up like a Christmas tree due to your whiz-bang glass cockpits, you might not get much utility out of it. However, as someone who flies a lot of nights...and has to do a lot of writing...the NVP was well worth the cost of a few drinks.