The truth is that Mesa is hiring a lot of guys for the D8 classes, and they need captains very badly. However, the Mesa Dash program continues to be one of the most difficult programs out there. I finished the training in July and saw that only 2 of the 7 captain applicants passed, both of whom had some serious 121 and 135 experience. This has been the trend for all recent classes (only a small percentage making it through). Fortunately for the guys in my class, they were given the chance to jump over to the right seat after being unsuccessful during captain training.
The only problem, lately, is that they have stopped allowing pilots to train for captain slots and then move to FO if their training is unsuccessful. It's kind of a "one shot" deal now.
My advice... fly in the right seat for a few months. If you meet the ATP mins, you can bid for captain immediately. Just give yourself a few months in the right seat. They are predicting VERY fast upgrades for the Dash program up in JFK (well less than a year).
The pay is much better as a captain... almost double... but you probably don't want to go through all the training and have something go wrong and be out of a job.
If you are VERY comfortable with flying turbine aircraft and have some 121 experience, you might do alright.
Hope this helps. Like is said... it's possible, but not too likely to go directly to the left seat.