from what I've heard from the guys at Beyond3D forums, DDR II is simply DDR SDRAM but designed to run at higher clock rates. it has one disadvantage (at least when it comes to video cards) and that is that it has an extended (from 2bit in DDR to 4 bit in DDR2) burst or something like that, which means a slightly higher latency which isn't good for video with all its fast z-buffering etc.
however that is the only shortcoming I know of, and it is only noticeable when comparing the two at the same clockspeed (even then it is negligeable). The guys at Beyond3D were talking about clockspeeds in the 500mhz range, which would not necessarily appear on motherboards anytime soon (unless it is really needed badly). In fact, AMD has no current plans to bring DDR II to the K7 core, but to leave it to the Hammer CPU, so if you want an AMD system with DDR II you'll have to wait for Hammer, at the very least. Intel has given no indication on when it will release a chipset supporting DDR II, but I suspect if it becomes viable, they may finally get rid of RDRAM (RDRAM has the ability to scale pretty darn high), at least in the consumer market.