They can't backup copy-protected game saves & aren't as convenient as a simple login upload/download on multiple PS3's. But to answer your question PS+ is about $50 a year (15 months if you buy now) for a list of things.
PS Plus is $50 (USD) per year, similar to Xbox LIVE.
While this is a cool feature, I still think PS Plus is in its infancy and it's not really worth investing in yet - I want to see more features/perks before I start paying.
Day 1 subscriber to PS+ and it has been worth every single penny. Beta's, Early Demo's, Free PSP Mini's, Free PS 1 Games, Free PSN Games (like Stacking, WipeOut HD etc), PS Qore, Discounts, Themes, Avatars, Full Game trials and now Cloud GameSaves with more to come as PS+ matures (rumored Cross Game Chat)...so yeah $50 imho is a deal for what is offered. All while being 100% optional