Flew them a couple of times to Europe and can't complain. Experienced some delays but as others have mentioned, I'm usually willing to take that risk reward in terms of price savings.
The two risks to be aware of IMO are:
1. As zkzkz mentions, because it's a smaller carrier with limited aircraft, you have a greater potential for longer delays due to issues and subsequent ripple effects.
2. They code share with Thomas Cook Airlines which IMO is crappier. You can avoid Thomas Cook flights though as Air Transat does denote which flights are code share.