I’ve pretty much switched from SquirrelMail to RoundCube, and so far so good. I do run into an issue when there is a huge HTML formatted mime attachment, but other than that, RoundCube is a great package.
One feature I really like is the address book. It took me awhile to start using this feature, only because I wanted to use LDAP for it. I tried a couple times to do so, but I was unable to make it so that new entries could be added via RoundCube.