The UI is much better than the gmail one, and the mobile apps are excellent. It supports tags or folders, depending on user preference (I prefer folders, so this is a huge advantage vs gmail.)
The spam filter is much better than gmail’s, at least for my account. Over the same corpus (my email went to both during a transition period), they both let zero spam through, but gmail was incorrectly blocking 10-30% of incoming email until I disabled its spam filter.