First I have to say I am a huge fan of the series even as it has evolved from being more RPG oriented to more of a first person shooter. The plots have been interesting, the characters well written, and most of all its been fun.
Lets start off with things that didn't go so well with part 3.
The ending: Without being spoilery all I can say what is wtf? You are given a bunch of back story and left with 3 options. The problem is that all 3 options lead to almost exactly the same ending sequence part of which makes no sense what so ever. I played through the ending sequence 3 times hoping that there would be some meaningful difference between them. There wasn't. After such an awesome game it was rather disappointing.
Difficulty: The enemy AI is much smarter in this game than in 1 or 2. They like to lob grenades behind your cover and execute flanking maneuvers. Some of the Reaper troops are rediculously hard to kill. I turned down the difficulty about a quarter way through the game because I got tired of dying. I'm here for the story.
Online play: This may be awesome but I'm not interested. The first person shooter aspect is my least favorite aspect of the game ad facing off wave after wave of Cerebus or Reaper troops doesn't sound especially appealing. I've read that you can't get the best endings without playing online but I'm not sure how that can make a difference. Half the stuff I read online about the game was BS anyway.
Planet Scanning. The monotonous tank missions from the first game are gone, as well as the resource scanning from the second. Good. There is still some scanning but its pretty minor. The catch now is that the more you scan the more you attract the Reapers, and when they show up you have to beat it out of the system or you die instantly. Planet scanning can reveal quest items and resources for the war so even though its still a bit tedious it is worth while.
And now the good:
The characters: All of the old characters are back and a few new ones have been added. There was lots of negative buzz about Freddie Prince Jr. joining the cast but I really liked his character James. It wasn't until I sat through the closing credits that I realized who was voicing him. The crew this time around is smaller than part 2 which had a huge crew. Everyone that lived through parts 1 and 2 are back in some fashion. My favorites like Garrus and Tali'Zora and part of the crew but most of the others have a secondary role as you run into them around the galaxy.
The writing: The writing is excellent as usual and is what really makes the game great. You really get know your crew, even the ones that don't run missions with you. There are lots of great little moments as they express concern over what's going on back on Earth and you have to keep them motivated and not lose hope.
The plot: I have only played through it once with my female Shepard paragon character. She has chosen most of the "Good" choices over the last few games but I did the Suicide mission in Part 2 without reading about it online and lost 2 members of my crew who were therefore absent from Part 3 (including the always hilarious Mordrin). In a month or two I'll probably play through the game with my male renegade Shepard and see how all of the different choices I've made in parts 1 and 2 change part 3.
Ending aside the game is excellent and I recommend it for anyone that loves a good space opera. The cut scene for the battle of earth was awe inspiring as the combined fleets of the galaxy show up in earth's orbit and engage. Even the small moments are just wow. It might lose some impact if you haven't played the earlier games but the characters are all introduced clearly and its easy to fall in love with them all over again.
If you like video games go out and buy it.