I've been using Office 2011 now for about a week instead of iWork. I'm a student and mainly use Pages, but I've been typing up assignments in Word this past week. Here are some thoughts on Word. (I don't really use Excel or PowerPoint).
Word lets you change the line spacing of text in tables, which is something that Pages wouldn't let you do. (This is a big thing for me, because my faculty has strict formatting guidelines).
It loads in about the same time as Pages did, however the word document gallery seems to bog down a little bit (cold start).
It looks more professional and feature packed than Pages. Pages looks very stark and bare (some would say that's good), but you don't have to dig through the menus as much in Word because it's in the Ribbon.
When you go into Full Screen View in Word, you can change the backgrounds, which is a nice feature. However, it doesn't show you the word count in this mode, which Pages does.
AUTOSAVE!!! This is a feature that iWork really should have, you shouldn't have to press Command + S every 10 min.
The application icons for Office are kinda boring. They have a nice 3D effect, but look kinda boring compared to the iWork icons.
No smooth scrolling in Word. Not a biggie, but it makes the application feel clunky when you move up and down a document.
Does anyone know how to change the font used when Word opens up?
Other than that, I'm quite pleased with Office 2011, but look forward to see what the next version of iWork will bring.