Most of the time they are OK. Their airport and on board service are certainly adequate and their on time record is reasonable. The big problem with Frontier occurs when something goes wrong - bad weather, mechanical issues, for example. If they have to cancel a flight, they do not have any backup aircraft and they will not transfer you to another airline. Usually, they have only one flight per day (or less). So if they have to cancel a flight, you can get stuck for a day or two. But honestly that does not happen very often.

If cost is your major criterion, and if it will not create a crisis if you don't get to your destination at your scheduled time, there is no reason to avoid them.