I've never used a service, but even if you do, I recommend "How to Form Your Own California Corporation" (http://www.nolo.com/products/how-to-form-your-own-california...) from Nolo for a good overview of the process if you're incorporating in California. It's not really very hard to do all the required filings yourself.
If you're planning to look for venture funding, you need guidance beyond what's covered in this book, but it's still a good intro.
I second that! Nolo has some wonderful resources available, both online and in print. If you wanna thumb through the book mentioned above there's usually a copy laying around your local Barnes and Nobles. I'd recommend reading it even if you plan on using a service later on. It's great info to have, and a very accessible read.
Only sidenote, in California the Secretary of State's rules, timelines and fees change all the time. Always double check anything you read on the SOS website http://www.sos.ca.gov/business/be/
If you're planning to look for venture funding, you need guidance beyond what's covered in this book, but it's still a good intro.