Can't speak for the iMac, but I've been using a mac mini for a few years. Have you considered one? They're neat little boxes, and leave you the option to upgrade to a better monitor as prices drop, or even just replace it if the monitor stops working.
Mostly the Mac has been a hit with me. It was purchashed as a means to an end (app development) but, whilst there is both a windows PC and MAC at arm's length, the MAC has become my workhorse of choice for day to day desktop duties, web surfing, email, etc.
Likes...
apple hardware is downright gorgeous.
OS/x is generally slicker than windows, boots faster, less buggy, 'just works'.
Dislikes...
Not always intuitive. There's no eject button on the optical drive for example, and it can be hard work finding the soft eject buttons. More than once I've found myself searching the net just to get a duff CD out. Later Mac Minis don't have an optical drive at all, which I suppose solves the problem, though I'm not sure it's my preferred solution.
Third party drivers, even though they support OS/X, are often a bit half-hearted. My Canon printer, for example, has auto duplex on windows, but not on Mac.
Hardware not always user-upgradeable. When the mac mini needed more memory I had to choose between paying the best part of £200 for Apple's service people to supply and install it, or to break the rules, prise it open with a walpaper scraper and do it myself for about £20 off amazon.
Even more than Windows, you may find yourself getting caught up in the perpetual upgrade cycle. In order to install some app you want, you might have to purchase a new version of OS/X, and once in a while a new OS/X might mean a new Mac.