I am in the U2 camp. Some people complain about the U2 beam having a bad donut hole, but I guess I got lucky on mine. There are various threads on people exchanging U2s with Surfire and getting one with a better beam.
The reasons I like the U2 are its versatility (6 levels), versatility (oh, did I already say that; it bears repeating like greenLED said), simplicity (simply turn a ring on the exterior to change levels), build (rugged with very high quality anodizing), brightness (rated up to 80 lumens but in reality much brighter), option to use a rechargeable battery, and beam quality and color.
The other Surefire flashlight that many are recommending is the L4. I think it is also a terrific flashlight. The L4 is shorter (about 5.1 inches for the L4 versus about 6.1 inches for the U2), has a narrower beam, is rated by Surefire at 100 lumens for the new model, and has good run time for such a bright light. (I didn't think the L4 was rated 100 lumens, but I just checked the Surefire website, and that is what it says). If you get the L4, I suggest you get a beamshaper from Surefire so you have the option for more flood ($12), and a tailcap from The Sandwhich Shoppe to give 2-stage capability. The Surefire beamshaper advertised for the E1R, E2E and E2O fits perfectly on the L4.
I find both the L4 and U2 to be too large for EDC in a pocket, and prefer other smaller non-Surefire flashlights for that purpose. If you do not carry the flashlight in a pocket, then I find the difference in size between the L4 and U2 to be not that big a deal. If you want to carry the flashlight in a pocket, then go for the L4.
Since the L4 is apparently rated 100 lumens maximum output just like the L2, the only reason I can think of to buy an L2, instead of an L4 with a two stage tailcap, is the extra cost for the latter.
Good luck.