We are using it internally, but haven't installed it at any client locations. The webaccess is a lot like the one from GroupWise 5.5 Enhancement Pack, only faster. That is, its a light-year ahead of the webaccess in 5.5 and 5.2 and it really quite speedy.
As for the native windows client, it seems much more stable than Enhancement Pack for those of us who were unstable. Those of us who weren't particularly unstable haven't seen much of a change, except for the interface enhancements.
The client looks alot like it did in 5.5, but has a few ease-of-use features I like, such as sorting folders and on-the-fly offline and caching modes. (caching mode makes GroupWise in limited bandwidth use even better than it was before).
Hope this helps!