I just finished this game on, heaven forbid, my PC today (Gasp!) and i have to say that I am very impressed. However, the problems the game does have are very noticeable. I'll talk about the worst aspects of the game first then work my way up to the better ones.
Although I think the PC version of the game was much less buggy than the 360 version, the graphical issues were still quite evident. The dynamic shadows look horrible on people's faces when you are talking to them and I personally had a couple issues with textures disappearing on me. Load times were also a bit tedious, although I'm pretty sure they were a lot better than they were on the 360.
Another problem I had with the game was the unremarkable worlds the side quests took place on. They were all basically the same planet just with different textures and atmospheric effects. The side quests themselves were not terribly exciting either but faired better than quests an MMORPG would typically send you on. Most of them just involved going to a planet and shooting some baddies but the game managed to shake up the scenarios just enough for me to motivate me to complete almost all of the side quests the game had to offer.
The combat system worked out quite well IMO. Nothing really felt broken or irritating at any time. Overall it was a well balanced, although largely unremarkable system that made the game enjoyable to play until the very end.
Now, if you've read this far into my post you have probably picked up on two things: I write way too much and it doesn't sound like I think Mass Effect is all that great of a game. On the contrary, I happen to like this game a lot. All those 9/10's those reviewers gave this game are completely justifiable.
The reason I think this is has to do with the absolutely amazing cinematic quality this game offers. Playing Mass Effect does feel like playing a 30 hours long movie that, at least for me, never gets boring. The amount of voice work and writing that has gone into this game is ridiculous. The programmers did an amazing job of keeping the conversations looking unique until the very end. You can easily avoid hearing a single line of repeated dialog as long as you do not ask the same person the same question twice. I find it amazing that the cut scenes and conversations, which are usually the most dull and laborious parts of an RPG one must endure, were actually the most impressive and interesting parts of the game.
The game's story is also very interesting and rekindled some of my interest in the sci-fi genre as a whole. I was afraid when I first heard about Mass Effect that it was basically going to be some cheesy Star Trek/Wars fan-fic or something to that effect. Although the game did have its share of cheesy moments, in the end the plot managed to retain a fair amount of originality. There was never a game changing plot twist like those featured in Bioshock or Portal but the story in Mass Effect came together in such a satisfying matter that your thirst for a good tale is thoroughly quenched by the time the end credits role. The ending even leaves room for a sequel but does so smoothly enough that it doesn't irk you like Crysis, Gears of War, Half-Life, Half-Life 2, Half-Life 2: Ep 1, Half-Life 2 Ep:2 or basically any other game you can think of that actively acknowledges that there is more to come after you "complete" the game. I couldn't say if Mass Effect has the best story a video game has ever had but I am having a tough time thinking of one that is better.
Finally, (thank God) when I was thinking about purchasing this game a couple of months ago shortly after it came out for the 360, I saw a lot of people debating whether Oblivion was better than Mass Effect or not. I can say that if you like action-RPG's you will like both games very much. Both games have their strengths and weaknesses. Oblivion's game world feels much larger and offers more to do than Mass Effect's "world" does, even though it spans about a third of the Milky Way. If you want to play a game for what seems like forever that feels bigger than life, you will like Oblivion more than Mass Effect. However, Oblivion really has nothing on Mass Effect's story and cinematic feel. If you want to play a game that gives you the satisfaction of watching an amazing movie and reading a compelling novel combined, then you will probably like Mass Effect more. When I finished playing Oblivion I felt exhausted but after playing Mass Effect I feel quite satisfied. In the end I cannot really say which game is better, but deep down I know I want it to be Mass Effect.
tl;dr - Mass Effect is awesome, buy it now, possibly as good or better than Oblivion.