I must agree with Airtime that your experiece with any airline greatly depends on the crew.
As for Air Transat, most of their pessengers are holders of an organized/package tour of which carriage on Air Transat is part of. So if you book your holiday with XYZ Vacations and they happen to have a contract with TS, you have no choice but to fly TS!
Charter airlines usually dont have spare aircraft sitting around on the ramp, so when the delay happens, or when there is some sort of irregular operation anywhere in the system (even weather related), it takes a long time to recover from it. For example, you are flying from Vancouver to Cozumel. The plane that was supposed to operate that flight was first scheduled for Montreal-Vancouver leg. However, due to the snowstorm out east, the flight was cancelled. So, Air Transat finds itself with no aircraft in Vancouver, and subsiquently no aircraft in Cozumel for the return trip. This one delay will have a chain of delays following it, until the operation returns to normal. Meanwile you know whats happening at airports -- you have hundreds of pissed off travelers whose vacation time is being shortened or who cant get back home on time.
With the introduction of A310's and retirement of L1011's, TS's reliability should substantially improve.
Hope this helps!