I had a Gen2 U2, and decided to have an emitter upgrade done by Nitroz, so I read every thread I could find, where I started to learn about the differences between the Gen1 and Gen2. I managed to track down a Gen1, and had it modded by Nitroz with an XHP-50. Holy smoke. It's all flood. and so bright that you can dial back the brightness more than you might think, thus increasing the runtime. Level 1 can be quite useful. It came with the plastic liner to block 18650 cells, but you can put the battery tube in a baggie in the freezer for an hour, shove your finger in the end of the liner, twist slightly, and it easily pulls out, attached to your finger.
The Gen2 got an XP-L2 (4000k). The output increase, as per Kestrel's observations, is not as dramatic as the change with the Gen1, but it is still a decent, somewhat throwy light that can run on an 18650. Both of these lights are definitely keepers.
