We fly Sunwing most frequently when heading to the Carribean. Our last flight (T.O to Holguin, Cuba) we did the upgrade - I had a nearly 2 year old Lap Baby and I wanted the extra space and the extra luggage allowance.
One other thing we did 'get' was priority check in (both in T.O and Holguin on our flight back). This was an awesome perk we didn't know about on the Holguin side.... anyone who has flown out of a small carribean airport... they had 2 desks for check in... 1 for Priority and 1 for every other passenger on the flight!
We have never had any issues (not saying there aren't any - but we have flown about 15+ times with sunwing and never had an airline related issue)