Apple TV is much cheaper.
Apple TV is cheap junk. I actually wanted one for airdisplay, but the dealbreaker was the atrocious menu system. All the crap channels you'll never use that you have to scroll past with the tedious toy remote control. Why can't apple allow you to hide the garbage that's only in the way to streamline the interface? The remote is another part of the Apple Disease, tiny and cute but horrible to have to use. Remember cheap TVs where the remote control only had up and down with no channel numbers, now apple has recreated the experience.
Adding the keyboard support was a recent "amazing" new feature. You actually think it's okay to try and enter content searches on a remote with a few buttons?
On my mac mini, I stream internet radio, TV from network websites that don't support ATV, watch DVDs (C2D mini with an internal drive), watch video with VLC in formats ATV can't handle, and store a large media library locally (640 gig internal HD). I also play games on the mini to enjoy my big screen experience. Considering the low cost of C2D minis, I can't imagine why anyone would buy an ATV, and considering the price premium on ATV2's for jailbreaking, it seems even ATV users don't like them.
Even at 6 times the price for a new one, a mini plugged into the TV is a much better option than an ATV.
iMac is also a complete dud. I've got no problem spending $1000 on a monitor, but there is no way in hell I'm having it permanently attached to a junky machine made out of laptop parts and sealed against upgrades. The $800 mini gives amazing speed out of the box, and then it's cheap and easy to give the mini a 256 gig SSD while keeping the 1TB HDD, so going the mini + ATD route gives me a better computer at a lower price and then in 2-3 years I can replace the mini and still have a perfectly good ATD on a brand new computer.
You can't even say the iMac gives you a sleek all-in-one any more since Apple wants to push everything out of the box anyway to save a few mm at the edges.
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The Mac mini currently has some features the iMac lacks; FireWire 800, HDMI, Line In and upgradeble memory. Apple's obsession with thin design could remove those.
There is a very good chance that at the next Mini refresh I'll end up buying a 2012 mini as my last Apple purchase until they reverse course (so my last Apple until hell freezes over).