I've been considering trying crossfit for awhile and I've been reading up on gyms (locally and not) and there appears to be multiple types. What you want to do is make sure your gym is run by people with more than just a crossfit background/certification. The most success I've seen long term (long term being the key idea) comes from trainers who not only follow the crossfit method, but also know how to use it progressively to promote long term change. Random workouts might be fun for awhile but unless you are progressing in your exercises, you won't progress in your fitness. Crossfit trainers with backgrounds in other areas (endurance coaching, PT, strength and conditioning, yoga, etc) appear to be more successful. They offer better form instruction, they offer MORE than just a canned WOD, they offer training plans and progression and they recognize that not everyone is exactly the same.

And yes, as a group, crossfit used to preach the Zone diet and now they have embraced Paleo. This varies by group though and I would seriously question any gym that made a diet a requirement (unless you've signed up for such advice as part of a challenge or a package or something).