If you are looking to become a good pilot, AMF isn't a bad place to go. Flying single pilot in a Metro, in heavy IFR, will make you a good pilot (or a dead pilot). As far as career advancement, however, it depends. If you are looking to go part 121, then AMF is almost completely useless. There is essentially no chance of getting to a major without going through a regional. It may have happened years ago, and there are stories about a "mystery person" that went to a major from AMF, but no one knows exactly who this mystery person is.

If you want to go corporate, then AMF has some real value. Guys are getting hired from AMF left and right right now for corporate flight departments. And is seems that once one guy gets in somewhere, he is bringing 3-4 of his buddies with him over the next couple of months.

As far as getting on with JetBlue/SouthWest/etc... - it will be REALLY hard to do from AMF. One recent captain that left AMF was told in an interview with JetBlue that he needed 121 time and jet time to be considered competitive with the applicants that they are currently getting. This person had more than 3000 hours of turboprop time at AMF with 2 type ratings and a total time of more than 6000 hours. The problem is that there are hundreds of regional captains that are also applying to JetBlue/SouthWest. They stand out in the hiring process, and the guy that has been flying boxes just doesn't measure up.

As far as Allegiant goes, I would do some research on the company and Frank Lorenzo. If Frank Lorenzo is affiliated with it, run away screaming!