I haven't flown them personally, but I know several people who have and said it was fine.
As mlatuchie noted, USA3000 used to be a charter airline only, but they have expanded to scheduled service running from the Northeast to Florida and a few other destinations. Their planes are relatively new, and they feed you on most flights.
The downside is that they don't have agreements with other airlines, so if your flight is cancelled you won't be booked on another carrier (this isn't specific to USA3000, and is a downside of all LCCs).