I've been using Carthook for about 2.5 months. Discount codes are a bit tricky and the checkout will not support gift cards. The cart builder interface is buggy (my cart went down over the Christmas holiday due to a still unresolved issue) and they don't notify customers of known issues. I recently inquired about issues with abandoned cart and got a note back saying they have been having issues and are phasing it out. Its $300+ a month for the service. Not worth it.
Pros:Easy cart set up, simple funnels interface, abandoned cart reporting and emails.
Cons:Most of the 'integrated' functionality doesn't work.