So far none. I've only had it for a few months, so if I encounter any problems I'll update this review.
So far I'm loving this machine. Produces great shots, the steam wand is excellent and it's very easy to use. It's also very easy to program and control. The water tank is an excellent design, and cleaning has so far been very easy. The machine came with a tamp which fits snugly under the hood (a nice feature), but I'm planning to get a calibrated 'click' tamp to get my presses more consistent. I'd strongly recommend this machine to others based on my experiences. My only concern is how it ages; I'm hoping to get three years out of it.