Unlike other commenters, I agree that there are some (arguably significant) things to complain about. The first one is price - tickets are quite expensive. I frequently travel Zurich HB -> Lugano. This is 200km and costs a whopping 120+ CHF round trip. Zurich -> Geneva, Zurich Bern are similarly expensive. However, it's a bit hard to fault them as Switzerland is an expensive country and perhaps the high prices keep the service good.
What I am less able to excuse them for is capacity issues, especially on weekend and Friday trains on popular routes in the Summer. That Zurich Lugano train is packed to the gills most weekends during the summer such that it's standing room only for most of the 2 hour ride. They need to add more trains or at least more cars.
Reliability is not something to complain about. The trains are punctual, that's for sure.