Gaikai, a PC game streaming service similar to OnLive, has just launched a Mass Effect 3 demo for your perusal. If you haven't tried Gaikai yet, it's basically an online cloud based demo service that allows you to buy the game through various means after the demonstration.
Unlike OnLive, you don't need any intrusive software or a registered account to do anything -- you just click the game, and provided that you have Java installed, it just works. So if you haven't already, go run that Mass Effect 3 demo, even if you have a lower end machine. Oh yea, and there's a ton of other games on there too (most notably the amazing Rayman Origins and overlooked Orcs Must Die)!
Gaikai Game Library [Gaikai]