Sometimes the consequence titles offer so many improvements that a re-play makes the first part not much fun anymore, because you miss this. This is not the case with Mass Effect! Even after you have the second part of or even played several times the third, Mass Effect 1 is still a beautiful game that's just fun and exciting.
Mass Effect has an exciting story and is nice to play. Although I find the second part even better, yet even Mass Effect 1 is on my list of favorite games with very high up there and I play it still like!