Their fleet is quite new, which is a definite plus. I've only flown them a few times, but thought the experience was pretty good. Yes, they're no frills, so be aware of the extras. I think the only extras we paid for were for the seat assignment and a checked bag each. (While not my favorite airline, Frontier, IMO, is far superior to some of the other cutrate airlines -- especially Allegiant)
Main drawback to me for Frontier is a local one. The baggage claim at Trenton (TTN) is horrible -- you wait awhile in a trailer for them to bring back your bags and they push them on old meat rollers (I'm not kidding) (Can anyone chime in if that has changed?)