Jeff, it all depends on where you want to dive. If you dove Lighthouse Reef, Glovers Reef, etc, then Ambergris is probably a little far to do those day after day from. You could get in a day trip to Lighthouse or Turneffe though. They typically leave at 6am and return around 5pm. The reef off of Ambergris wasn't bad, but it just wasn't as good as the wall dives of Lighthouse Reef. I've heard the same from everyone else I've talked to about this.
If you're looking for some place to dive and have a relaxing vacation, then Ambergris is the place. It's real laid back. Not much to do other than kick back and relax, but it's great for that! It's also close enough and has access to a lot of other activities on the mainland. You can do trips to the ruins, cave tubing, etc. Keep in mind though that from San Pedro Town, you'll be in for a long day every time you do those trips. They usually fly you out around 7am and you don't return until around 6pm.
So, it all depends on what you're looking for.