Yes, we've been with them for all but 2 of the last 28 years. We have always had excellent care there. I don't think we've had anything terribly exotic or complex to treat though.
The thing people who don't like them complain about is that you can only see doctors they employ. There's no such thing as "out of network", so you can't get a referral to an endocrinologist and then go see the independent one your neighbor recommends. If you have a thyroid problem, you'll be seeing someone at a Kaiser office. The good thing is that all your medical records are available to all your care providers, so your primary care doc will know everything that the endocrinologist does to treat you, and the pharmacies will know about all your prescriptions.