I have been with Mesa for almost 2 years. I have heard a lot of negative stories from friends who were here in the past, so I came in with a really low bar of expectations all in the name of quick hours.
That being said, is Mesa has not disappointed me. I fly A LOT, I had the opportunity to upgrade at just over a year (flying the CRJ) and I like a lot of the people I fly with (yes there are those who we don't like, but you'll find that anywhere).
Mesa is still going to get you to do things that are in the grey area of legality, or tell you that the Pilot Working Agreement says one thing when it soys something different. They are just trying to get their flying covered with the least almost of effort.
My advice: Come to Mesa, especially if you are under 50 years old. That give you time to get you hours and move onto a major. If you do decide to come here, know the contract, know there LOU's and LOA's of the contract too. The real key, is take this serious, but have FUN with this. This is a good place to fly to build time and make friends, and maybe one the culture will improve as more and more younger pilots come in and stay a while.