Mass Effect 1 was fine, but got into your typical RPG "grind" a little too much for my tastes. Didn't stop me from enjoying a play and a replay though. 🙂
ME2 has a MUCH better story IMO, fewer repetitive grind missions, less emphasis on credits and done with the stupid inventory system (selling back every item I found after every mission, micro-managing weapon upgrades for specific missions against specific opponents, ugh). And the 2 DLC missions for ME2 are well worth it IMO, especially "Lair of the Shadow Broker". You should hear about him in a couple of more hours if you haven't already. 🙂
Also, the ME2 castes have more effect on gameplay, better AI as well.