Jerseyguy summed it up pretty well. I would just add that depending on your route, there will probably be fewer options/alternatives available if something goes wrong. Many of F9's routes are once daily or even 2-3 times weekly. That is the main thing you're giving up, IMO. Of course, if you're flying out of/into DEN, this might be less of an issue.

However, also depending on your route, F9 might actually be a better option. I'd certainly choose it over a legacy that operates with a regional carrier because of the overhead bin space, the increased seat width, and the ability to watch live TV (even if you choose not to pay, you can watch for free on the tarmac and for the first 15-20 minutes in the air). Also, call me silly, but I actually like the idea of charging for soda, because if I am thirsty, I appreciate that I get the full can, but if I'm not, then I don't have to worry about paying attention to the FA coming down the aisle.

Keep in mind that Frontier used to be classified as a "major" airline, unlike Spirit and Allegiant which were built as penny-pinching soulless companies from the beginning. So it'll definitely feel more like WN or B6 than Spirit/Allegiant. Like Jerseyguy said, though, this is all subject to change in the coming months. For now, it's a decent airline.