1) Allows slower PCs to run games they couldn't otherwise and allows gamers who have lower-end PCs to play those games.
2) Easy access to a lot of great games (provided they are supported by the system)
3) Community content and aspects.
1) Very strict DRM. The system works more like renting than owning.
2) Subscription and cost of games. Games might be cheaper on a general scale (doubt it) but the fact that a monthly subscription is needed decreases value at least to me.
3) Internet Access. Without it, you can't use the service or play the games because everything is hosted on their servers.
Short: I'd rather just upgrade my system and pirate. X|