Ok, here's how it is. An iMac (which I have, by the way) is a good computer; don't let people tell you otherwise. However, if it's for school, do you really want to have a desktop? Also, tip: DON'T BUY NEW! Go to a resale computer store (there are even some Apple specific ones), or buy refurbished. You can get hundreds off that way. And while you could build a hackintosh, I wouldn't suggest it, because if you have certain problems with it, you'll have limited support options. Anyway, above all, the iMac is a good computer, but it isn't portable, by all means.
It's also very pretty. ;)
EDIT: Also, don't pay attention to the polls. Many people will vote no just because they use Windows and are biased towards it. Only a selective few on the BLF will actually give you their unbiased opinion.