No problems so far with my N91 4GB, except that I would like to have more RAM memory available. So often while using the in-built Web browser (and no other app running) I get "Memory full" warning , so multitasking with browser connected is almost impossible for me.
Other than that it's a great phone, excellent music quality , stylish design, reliable construction, great camera (much better than my N70), good battery life etc. The display and keypad are OK, AFAIC.
I use Office Suite application for handling Word ane Excell docs. It's slow but it does the job