As others have said, CS has been slow and complaints/problems seemed to get dropped all too often. Their new support site
http://getsatisfaction.com/kobo seems to quickly attend to these issues and seem especially quick to respond when there is an open ticket that has been ignored.
I have purchased 52 books from the Kobo store, and while I have had CS issues (all of which were resolved) none of them have been related to problems buying through the kobobooks.com store.