i used MS office at school and got REALLY used to it. then i used openoffice at home, and it drove me nuts. there were a couple of good features in OO, but it lacked the polish that MS has. i will stress that this is because i'm really used to the MS suite.
some of the weaknesses i found:
-lack of built-in powerpoint templates/clipart
-can't use scroll wheel in powerpoint to move between slides.
-word processor doesn't have a split pane function
-word processor makes it difficult to remove links after the fact
-spreadsheet functions use ; instead of , (took me forever to figure that out, not really a weakness)
-spreadsheet graphs/charts much more difficult to manipulate. (basic charts are ok)
-in excel, i always thought it was weird how it highlighted cells when you cut/copy, but it turns out to be really useful when OO doesnt have it
these issues are minor, but the overall feel just made me buy MS office anyway.
but then, you should try OO anyway since its free!