Personally I couldn't get into ME until ME2 came out. I felt the character writing of the first was a bit stoic and not as in-depth. It made it hard for me to really care for them. Where it did excel over ME2 was the overarching story and world building fiction.
ME2 made it easier for me to care for events because it concentrated on the characters with amazing writing. Suddenly characters from the first game that I didn't care for came alive and felt real.
ME3 used the second games focus on the "human" aspect of events and characters but in a much larger and incredibly darker way. I personally loved 99% of it, I've never been as affected by a video game as I was with ME3. Yeah the ending is literally "WTF" flabbegastingly bad, but the journey getting there was freaking epic.
Overall I like to think of Mass Effect as one game. Each nicely balances the others out in terms of focus and overall plot development. I can quite easily say that this series is the best game within the last decade- possibly two. If they do fix the ending to be what it should be then I'll say it is my favourite gaming franchise of all time, even ahead of games like the 90's Xenogears, FF6, 7, 8 and 9, and other great JRPGs in their prime.