I fly JB almost exclusively. I like flying with them. Yes, they can change flights, but I usually know about it in plenty of time. If I fly out of either TF Green or Worcester, I know that I could have an issue....there is one flight daily out of Worc, and several out of Green. There may not be any choice if my flight is cancelled or the time changed. If an airline changes your flight time by more than a few hours, they are supposed to issue a credit, if you ask for it. And if cancelled, a full refund is in order.
The only issue I have with JB, and it's not really their issue, is I'm never sure where I'll be flying in or out of when using Worcester. But, that's an airport issue that is being fixed..albeit slowly. I do get taken to, or from, Worcester if there is a change to Boston, or even to Providence. It's just a pain.
But, all in all, I enjoy them. The fares are usually competitive, the staff is good, I still get a snack in flight, and arrivals and departures are close to on-time.