One of the best interviews I had included talking with the various users I would be working with on a regular basis. This helped me understand how much the company cared about delivering the right product to our users. The job was for a Reporting/Database Analyst and I loved working for that company and I would have retired there; but I moved to another state.
One of the worst interview I had included a question on the phone that went like this "Are you a hard worker?" Ummm no, I'm lazy but please give me the job! I have had onsite interviews where I took certification exams, other types of test, etc. It's not the greatest assessment tool, but it's alright. I prefer when the company has their own testing environment as opposed to using a Certification Exam (since I don't work with XML all those questions were annoying!).
I've also had interviews were the person on the phone asks a ridiculous question for which one would have had to memorize the text book and then tells me I'm wrong and tells me the right answer afterwards. Needless to say, I rejected going for the onsite interview. One humiliation is enough right?
Another thing to look for is to see if you keep getting repeated questions from different interviewers. This tells me that there is some communication breakup somewhere in the chain. When I run interviews, I make sure that the entire team has the set of technical questions we will review and then we tell each other which questions were asked. That way when we want to ask a similar question we ask for more depth.
It is also a good sign when the interviewers are relaxed and may ask some informal questions or have a sense of humor. You never want to work in a place that is run like a concentration camp where the death penalty is issued for smiling!
My two cents!