I've done it once or twice thanks to clients who insisted I travel on the absolute lowest available fare. Not that bad. The aircraft are newer and in better shape than many in the majors' fleets, and the service levels in economy are comparable to, say, Northwest (e.g. nothing but a soft drink) without the side dose of Bitter Employee Syndrome. If you don't care about major-carrier miles -- and now you can even earn through RapidRewards, I guess -- I say go for it. No worse than some big network carriers.