This is a subject that it can be hard to get a good answer on...
But, yes, rakeback is a good thing. It's like getting a small bonus every time you play. Don't sign up for any new poker sites until you understand rakeback better.
I only have it at a small site (because I'd signed up with all of the major sites before understanding it), and it is certainly a good thing (and the site should be happy to know it causes me to play on their site a heck of a lot more than I would otherwise).
You'll probably have to do some research in multiple places as you won't find a nice simple FAQ that explains everything (or at least I never found one... please post the link if you find it).
The
Affiliates/Rakeback forum on this site has a lot of info -- not all of it helpful, but you should read through a lot of it anyway to get a sense of some of the issues involved.
Rake Rebate Review is another place to get info.
One thing to remember is that rakeback is usually offered by "affiliates". The affiliates can offer the best information, but they are also motivated to get your business, and not all of them are trustworthy, so keep some grains of salt handy...