We have found it cheaper than Riteway (not much in it though) and find it very useful for all the heavy staples (water, rum etc). Its rather nice to arrive on the yacht after a full day of traveling and find all the heavy provisions already stowed away.
...but we always head over to Riteway ourselves for fresh provisions.
The only downside is that the Sunsail list is quite limited...
For what it is worth, when we were there two weeks ago we had always in the past we always realized and thought that One Mart was cheapest for our provisioning, but this year my mom came and she is the shopping queen. The majority of items were cheaper at Riteway including the liquor. We order from Caribbean Cellars and when she compared prices, most of the stuff we drink was about 15% cheaper at Riteway than what we paid for at Caribbean Cellars. Not really that big of deal but when we all save for two years to make this vacation happen, every little bit helps us.