I like that it's easy to use, has good customer support (even though they're not always available), it's free for small businesses and there are no ads! It can sometimes be hard to find what you need from their search feature if all your options aren't displayed.
The UI is also quite outdated in some areas, but this isn't something we've come across as an issue. We needed a way to keep track of our server health so we could better know when to call out or have someone get into work early. This solved that problem very well!