Yes, I like roundcube too, but it is in very early stages of development. For instance, AFAIK, it does no allow a user to change their own mail password. Also, deleting or moving a long list of messages is tedious. And, it does not allow for setting up a auto-reply. Things like that...
While I see lots of votes for Roundcube, I'm now very wary of it.. I had recently loaded it on my FC6 (Linux) box (perhaps 2 months ago) and started to configure it until it wanted a newer version of PHP (I was running a slightly lower 5.x version than it wanted). Anyway, I never got around to fully upgrading my PHP version to make it happy.. Anyway, about a month goes by and I started noticing odd stuff on the server.. To make a long story short there was a security issue in some of the RoundCube code that allowed someone to exploit a hole in the roundcube code I had and they loaded up their hacking code to try to exploit my Linux server. Anyway, I shut them down but have very little interest in using something that bleeding edge if the possibility remains that they can hack their way into a backdoor to gain access..
Obviously FreeBSD offers some benefit in this case if you're using jails but it may not be fool proof perhaps.
In the meantime, I'll stick with my tried & true Squirrelmail which I've never had any problems with (security or otherwise).
I will admit that Roundcube has the eye-candy thing going for it but it's not enough in my opinion..