I like that it's completely customizable, so you can make almost anything work how YOU want it in your organization! It has its quirks at times (especially when using an external LDAP directory), but nothing terrible about it. The team behind EGroupWare are very helpful if they don't have what you're looking for already built-in - which isn't often with their product.
As previously stated though there aren't many things we've needed that weren't easily solved by building something customβ¦