I love AA/AAA form factor
I've got nothing against either you or the TK40, but to say that light is a AA/AAA form is just . . . well . . .
It's more like a D form factor, judging from its dimensions (I don't have one, so no, I can't be sure). As far as the AA form factor, the Javelin is a much better example. It's thin, though a bit long for pocket carry.
Like I said, I don't have the TK40 but have read lots about it, both good and bad. I'll speak to the less publicized Javelin, which I do have. It's really nice, though it would be much less impressive to a newbie, as LittleBill said. Since it uses the newest emitter, the XP-G R5, it's a lot of light for just two batteries (nothing compared to the TK40, but what do you expect?). Three modes that are accessible with one hand without having to mess with turning the head. The finish of it is just impeccable and it has a very handy, well-designed pocket clip. Even with the the extender added, the clip holds well. There's anti-roll on the light too. The Javelin is pretty low maintenance and is good to go right out of the box. Come to think of it, I don't remember reading much about people having to mod the TK40 either (I read a lot about it before deciding against it).
You're probably thinking of the earlier Nitecore D10s that had some piston issues and needed lube, or the various and sundry DX and Kaidomain lights that need service/soldering just to get them to work. Neither of these lights will leave you in the dark and you'll get great customer service if you choose the right vendor, especially if you go with the
flashlightconnection or
4sevens.
Are you sure you don't want to be comparing the TK20 to the Javelin? They both use the same power source, though the Javelin is thinner. Not wimpy, just thinner walls and not rubber sleeve around the body. It's not too fair to compare the TK40 to the Javelin since they are really designed for two very different types of illumination. I own both these lights. The Javelin still wins mostly for its form factor (it's thinner than the TK20). At least they both tailstand. Then again, so does the TK40. It really depends on what you're looking for, again. The TK20 throws pretty well but the Javelin puts out more light and has a lower low mode. The TK20 has only two modes and they're spaced too closely to really be of much use. Also, MY TK20 whines. It's too much for my ears. My Javelin whines only on its medium mode.
No hard feelings, Bill. :tinfoil: