Each situation is different, but sometimes slowing the user down can reduce errors/improve accuracy. It's not that the user experience is "too good," but moreso that certain UX and design decisions can make the outcome of an interaction less than desirable. Luke Wroblewski talks a bit about this in his book Web Form Design (Rosenfeld Media). He discusses the placement of labels with regard to fields and how each option affects completion in terms of speed and accuracy. O'Reilly also uses similar logic in the creation of their Head First series. They use Comic Sans (and other design tricks) to slow down the reading experience as studies have shown it aids with retention. There's nothing wrong with slowing a user down if it's in their best interest.