Is it an enjoyable game before the ending? is it a good game? Does the story have good moments?
A spoiler-free version of how I see the game. The ending ruins part of the fun, but it doesn't change the fact that the whole experience from game 1 to game 3 (minus the final 10 minutes) is exceptional.
I like the balance between actually playing (i.e. shooting stuff, scanning planets or stuff like that) and watching cut-scenes/talking to people. As a shooter, the second one is the best in my view, as the fights (on normal difficulty or whatever it is called) are not really that hard but still challenging. Part 1 is a pain in the arse in terms of the interface they used. Part 3 has too much of that "Well, you've finished one part of the mission/found an important item. Let's spawn another couple of dozen enemies in areas you've already cleared before so you don't get bored"-gameplay (especially in the final mission).
I also loved the stories that were being told and that you get to make decisions that have an influence on what happens next or in the future. People will complain that there are no "huges" decisions that completely change the course of the game (or the ending), but the way Bioware have done this in all three games is still something I haven't seen in any other game. In games like Skyrim or other open-world RPGs you do tons of stuff that would probably change the world. Yet, it barely gets mentioned. In ME it at least has some influence on what is happening.
The best thing about the games is that Bioware have somehow managed to put me under a spell or something. Otherwise, I cannot explain how I got so attached to the characters. Characters that are basically a shitload of pixels on my computer screen. Yet, I get goosebumps if they have to make an important decision or if they die. At the same time, it feels great, if I meet someone I've met before and simply have a chat with them. About what they're doing now, about how meeting Shepard has changed them. That's why for me all three games (regardless of the ending) have been a truly memorable experience.