ME2 is definitely worth it, but if you start playing it without using an ME1 saved game it assumes, by default, that you did everything the "Renegade" way (which I personally do not like). If you have a PC there are some savegame creators/editors you can use to get around this problem, but if you're on a console I'd suggest playing ME1 (which is great in and of itself, although certainly not perfect).