We quite like Jet2 as they fly to an ever increasing variety of destinations from our local airport (Leeds Bradford - their home base) and in our experience operate pretty much on time. Unless you are lucky though they are not a budget airline when you calculate total ticket costs as opposed to the headline-catching base fares. I guess you generally tend to pay a reasonable fare for the convenience they offer.
We have recent experience of their online check-in process, having used it four times since June. Checking in and printing your boarding passes is simplicity itself, as is providing passport info (that can be done at any time online from point of booking). However, the system can fall down when you get to the airport as, with Jet2 charging you for checking in online but critically less than for doing so at the airport, many passengers opt to do so and, if you have any hold baggage, you than have to queue at the airport at one of the dedicated "online check-in desks" to have your passports and self-printed boarding passes checked and your cases weighed. The queue for these desks can be minimal, but equally it can be as long as the normal check-in one. One other point is that at the online check-in desks the staff sometimes reprint your boarding passes if they consider the quality of the ones you "prepared earlier" to be inadequate.