I personally consider ME2 to be "The Empire Strikes back" of the trilogy.
Absolutely agree. I played ME2 & ME3 and just now started with ME1. While 1 is great, I find it to be lacking in comparison to 2 -- especially with design choices.
But since you're just starting the series, I'd absolutely start with 1.
I play on the PS3, so I downloaded ME1 when it became available in December, just completed it and moved on to ME2 with the same character. I've already played through ME2 -> ME3 twice, so this will be my first play through the entire trilogy.
About 1/3 of the way into the ME2 portion of the story, I've already noticed the storyline change a little based on things I did in ME1 (as opposed to using ME2 as the starting point on my previous two play-throughs). I'll be interested to see how much more of the ME2 and ME3 storylines are impacted.
I liked ME1, but I do think ME2 and ME3 are better games. The look of the games alone became much more refined and gritty, and the story itself moved along much better in the latter two games.
That said, I still loved being able to go back and see how characters and other things were introduced into the storyline. As someone who started ME2 without any prior exposure to the story, playing through ME1 first helped put things like the distrust of Cerberus, disgust with the Council and evolution of the Reapers into much better context.
Another thing - and this was subtle but helped enhance the enjoyment of the game - was the score. The Mass Effect series contains an impressive musical score - especially ME3 - and it was cool to hear certain themes in the score carry over from one game to the next. One of my favorites was how they used Vigil's theme (heard in the title and menu screens of ME1) in some of the main transitional cutscenes in ME2 and ME3.