I've been with Kaiser for about a grillion years through my employers. (over 20 now, I think). Couldn't be more satisfied.
The main benefit in my mind is one-stop-shopping, as mentioned earlier. Regular doctor, gyn, tests, pharmacy, specialists, physical therapy all in one hospital. Parking is a bit of an issue but where isn't it?
No matter what doctor I'm seeing he / she can call up my complete record and review it -- I don't have to remember everything myself and the Dr. doesn't miss anything someone else did.
Another thing I appreciate is automated notices of what I need to do next -- is it time for a mamogram? They let me know. Every time I visit I get a printout with a list of dates when I had standard tests, and when those tests are due again. These include cholesterol screening, pap, tetanus shot, etc. etc. When I turned 50 they sent me a little kit in the mail to take a sample and test for colon cancer.
I like the advice nurse too -- I call and she / he reviews symptoms and either suggests i come in or directs me to more resources, gives me self-care advice, etc.
I'd be really disappointed if my employer stopped offering Kaiser.