I think they're definitely a step up from Spirit/Frontier/Ryanair, I'd say they're more similar to Southwest. When we flew them last year from TIJ-MEX it was pretty smooth sailing, except that they wouldn't let my daughter sit in the carseat we brought her because the tag saying that it was approved for aircraft was in English instead of Spanish. That was frustrating, but everything else went fine. On the way back we flew Interjet, and I would definitely prefer them over Volaris.