We have had nothing but great experiences with them both. There are extra add ons, but no more than we paid when we flew American coming home from San Juan in January. On Spirit you get a free personal item which is pretty large. We are taking a 4 day trip in a little bit, and we each are just bringing our personal item (large back pack). The $25 check bag fee seems pretty standard. When we flew last year with Frontier, we were able to bring one standard carry on, plus a personal item for free. They charged the same for checked bags, as I have seen from most other carriers, if you pay before you get to the airport. I think the bad feelings come from charging for upgraded seats. I can't say for sure as we have always been taken such good care of. The only real extra we paid for on either was to get our son a seat with extra leg room (he is 6'4). It was the best $15 we spent on that trip.
~Tink~