I've never used an online service for scanning before, but I don't have a slide scanner, so...
It was ok. Service was a little slow, but probably better now that the Groupon avalanche seems to have passed. I have two complaints, one, that the scans at their regular resolution are pretty small for this day and age when cameras regularly bost 12 Mp or more; and second, their website was so incredibly slow, even on a fairly modern MacBook, that it was almost unusable. It made checking on order status and downloading files from the online gallery almost impossible.
For a business that is based in tech, that is inexcusable. They really need to take another look at their business model, because they are not up to speed.