Booked Iberia's budget airline, Vueling last winter for travel within Spain this Sept/Oct. Read lots of reviews, there were not a lot of serious complaints then. We had booked 4 flights. Got to airport for first 8 AM flight, told us we were on an evening flight. I had printed out boarding passes with 8 AM, their computer had different info. We would not agree to the change and after waiting awhile they honored our AM flight. We received an email one day before our next flight that they changed the time from AM to PM. We'd had connecting flights to make the next day so we're kind of worried as they were postponing our flight. Our RS tour guide told us their service had been declining the last few months. We had to accept the flight change, went online to print boarding passes and they charged us $42 more to book our seats. Now we are concerned about our last two flights but luckily they were not changed. But 50% of our flights were changed along with paying more money.
If you managed to actually travel it went not so bad for Vueling standards. This summer they had big troubles in a few Italian airports; some flights were cancelled and their customer service being what it is, they left a long trail of disgruntled customers rioting in airport. They got to the point that ENAC, the Italian civil aviation authority, got extremely annoyed and threatened to revoke their flying license over Italy if they could not get their act together.
I flew to Portugal on Vueling this summer. Departure time had changed about 45 minutes (later) with no warning. Tried to check in and got pretty poor customer service. They said they would start 2 hours before the flight, but ended up making us wait until about an hour before the flight. Let's say they make me a little nervous. Good luck.
When paying discounted prices you cannot expect the service level of a legacy airline but you should get at least min service.