The reason they force you to register is so that you can qualify for assorted things... for example, you can't do parajumping unless you have passed jump school with high enough marks. Nor can you select the sniper rifle unless you first hit a certain goal with iron sights on an M16 (36 of 40 targets at assorted distances), then you go through sniper school to boot.
Second, it is a lot of fun. The combat is great, the graphics are good, the netplay is solid and lag-free for the most part (A lot better than the current compitition... miles ahead of Battlefield 1942, for example), the weapons are varied and fun, and there are a lot of tactics involved in play. There aren't many games where you will hear people say "Get down, I'm laying covering fire on the bunker", or "Someone set up a crossfire with me in the pump room", but this is one of them. There are a lot of neat touches... being able to go prone (not just crouch), being able to roll while you're prone, guns jamming, leaning side to side, extendable bipods on some weapons to increase accuracy, actualy having to time shots with your breathing paterns when trying for accurate shots, grenades that are suitably usefull, etc.
I'd give it two thumbs up, myself. And it's free to boot.