impressive?
K2 is old technology. i would also be concerned the glowing tailcap would always be draining the batteries. might want to look at a cree or seoul based light. if you want a nice bright 3xAAA light buy a modded coast focusing led-lenser from flashkat, or one of the dealextreme lights to save cash.
K2 is indeed old technology. But I find it pretty good still for flashlight applications. It is certainly better than the Luxeon 1watt and 3watt which are just flooding the mainstream consumer market right now.
One thing I really like is K2's max light output is pretty high (around 80 to 120 lumens). It's low output puts it around 20 lumens. At 20 lumens, my RR K2 3AA headlight runs for 20+ hours (2500 NiMH). Now that's pretty good runtime. How many manistream (available in stores) lights with 20 lumen output can claim 20+ hour runtime and at the same time can spit out 80+ lumens if needed on demand. I like that option.
Definitely, Cree is the best at the moment but the K2 ain't that bad. Since the Cree isn't really that widely available in brick and mortar stores right now, the K2 is probably the best LED available on the mainstream market, in terms of light output.