I like Mass Effect 1 and 2 better than Dragon Age, but I generally favor scifi over fantasy. All the big differences between the two have been mentioned by above posters, but there is one thing I didn't notice mentioned, or at least not explained completely, which is my favorite part of Mass Effect.
In your standard RPG when it's your character's turn to talk, you have a set of options to choose from. These options are exactly what your character is saying and you never hear your character say it, because it would get old real quick to hear your character read off what you just selected.
Mass Effect doesn't do it this way. When it's your turn to talk you have some options that are just the gist, or general feel of what your character will say. Once you make a selection your character says whatever the creators have scripted to fit with the option you selected. Doing the dialogue this way makes the entire experience more cinematic and really sucks you into the story. This aspect alone puts these games at the top of my RPG list and I really wish more games would steal the idea.
I also didn't see the annoying time trap side of these games mentioned above. Mass Effect 1 is loaded with really awful side quests which provide much needed resources and almost no fun. Mass Effect 2 made this a little better by introducing resource scanning of planets, which is also not fun, but not as time consuming to get resources when you need them.
In the realm of gameplay it doesn't compare to Deus Ex, but nothing I've played ever really does. Now that I'm thinking about it, I want to get my hands on it again and give it another play through.
All of that said, both Mass Effect 1 and 2 are definitely worth playing.
