I have a Nokia 9500 myself and tried using it for GtD and have also needed to drop it almost completely. As with the E90 you can't sync categories correctly so you end up with a mess. The screen size and the quality key board make it appear like a great tool for GtD, if it weren't for the categories issue.
Given that the keyboard on my Commie is actually really nice, I did try implementing GtD using only the phone, i.e. without trying to sync to the PC. The font for the calendar app was too large, so I switched to Projekt, a really good (standalone!) outline app for Symbian. It has lots of features and lots of font sizes (for the N9500 at least). Nevertheless, I stopped using it because the phone screen was too small for me to work with it comfortably. The screen on the E90 seems bigger though, so who knows.
Now I have switched to a paper planner and I have noticed that having so much (paper) real estate, and its flexibility, is something that really goes with me. So possibly it wasn't that the screen size was so much of a problem, but simply that I need something like paper to work with.
I still use the phone for the shorter context lists that I want to carry with me all the time, but that I don't consult 20 times a day: @home, @errands, @shopping list, @web, @Agenda - partner, @email private, @call private. For this it works brilliantly. Also it serves as my UCT.
I am thinking of switching to the E90, so if you find a good GtD implementation for it, please let me know. Hope you enjoy the phone...