I worked at Jefferson. First job out of school, so I wasn't the smartest guy to take that crap ass contract, but once I signed, I did my 15 month stint (the life of the contract at the time) and got out. I have never worked at Smiley or Monarch, but I know some dentists at both.
First off, all associateships come with their own set of flaws and advantages...all of them.
It's been a couple years since I worked at Jefferson, but if you ask me questions about it I'll tell you about my experience.
Pros: Guarantee minimum per month, good assistants, good dental materials used, focus on clinical dentistry (instead of business), large group so other dentists in group can be referred to for complicated cases
Cons: Managers at some offices can be pushy, especially in the beginning you will go to many different offices each week and can be rotated without notice, equipment at some offices is wearing down, no hygienists, multiple patients at a time with a lot of running from chair to chair, some diagnosis and treatment decisions can be mandated by clinical directors (i.e. I don't think this needs a post and core, but director says do the post and core)