I use Gmail privately and
CrossBox at work.
I must admit that crossbox blew us away with the number of cool features and a really slick GUI. Unfortunately, it's not free but we had some luck to get it for free via our hosting provider which offers it as one of the webmail options.
I used to use Roundcube a lot but now it's even more apparent that it lacks many functionalities to be called a modern webmail client.