One thing I don't like is that their Android client which supports push notifications (a feature I personally valued in Gmail) also has some weird UX.

For example, the message actions bar within the "view message" pane. At first, I wasn't aware it existed. I thought you had to archive/delete messages from the inbox by swiping it! I only discovered it by accident when my finger missed a link in an email. It seems like it appears when you tap somewhere in the message itself, and it appears over the top of the bar that holds the message title and the back arrow.

Very silly in that regard, but otherwise quite solid. Their servers, service, and desktop offering are great and I found their pricing quite reasonable.