I recently discovered RoundCube webmail and am now in the middle of testing and offering it to all users on my system. (approx. 50) Now I use Squirrelmail (what else would you say perhaps) but the new, fresh look and the way the userinterface works feels great.
The only really thing I miss now, during the migration, is a tool to convert addressbooks of users from squirrel to roundcube and for first users it should me nice that they have to set their primary identity first before reading/posting mail.
Further, keep up the good work.
PS My RoundCube is at
http://webmail2.linuxpro.nl