I got my Collector's edition for PC yesterday. Freaking awesome game, even more epic story.
And if you can't save Wrex in the first game, you need to allocate more skill points to charm or intimidate. There's an achievement for you if you do. Thankfully in ME2 you don't have to waste precious squad points on charm or intimidate. Rather, the game opens up charm/intimidate options as you progress, if you do enough paragon stuff you'll have all paragon conversation options opened but not be able to do renegade.
I also love the new engineer powers, plus a return of AI Hacking, even though it's rather limited since you don't face as many synthetics as you did in the first game. Its also nice the way Incinerate and Cryo Blast work when you don't have open line of sight on the enemy. You can hook it around cover to smack them what-for.
I also like the absence of the eleventybillion weapons and armors from the first game. Inventory management sucks, and Bioware realized that. All you really loot anymore is credits, Upgrade schematics, ammo, and resources. Surveying for Resources also has a substantial benefit.
As for buying ME2 on PC, I bought ME1 on PC just so I had a savegame I could use with the sequel. Glad I did too, ME1 was a breeze to play with Mouse and Keyboard, and the way the weapons have changed in ME2, I'm even more happy to have my preferred controllers. Not only that, having hotkeys for each special attack is a MUST for me since as an engineer, I find myself heavily relying on them. Oh, and I LOVE the combat drone. It doesn't do much damage, but it has saved my life a couple times already.