So far, the app I've seen that seems to have the best UI is Sherpath.  Very Clean and well integrated.  However, (doesn't it seem like there is always a however?) there are a few problems - at least one keeping me from rolling it out:
1) No integration between LDAP and Sherpath.  They are working on this, but it isn't there yet.  Right now you have to duplicate your accounts within Sherpath.
2) 90+% of the app is English.  Still, there are a few screens with some Frnech on them.
3) Help is totally in French.
With the LDAP integration, I would roll it out now and finish the translation myself.  It is not, though, so I have held off on groupware.
PHProjekt looks like a quite nice product as well, but the UI just doesn't appear to be professional/business oriented enough for me.  I really like the simple Helpdesk functionality, though.
Kevin