When I bought my big iMac (27 inch 3.4ghz) it developed a rather obnoxious blue line down the centre of the screen. I took a pic with my phone and then took it into the shop where I bought it .. they replaced the whole screen. Apparently this had been a known problem with the iMacs but apple will replace it without any arguement.
Ever since then that computer has been great. It's a mid 2011 model .. and I own three of them. One was a swap for my 17inch MacBook Pro which i toasted, and the other was ex-demo from my local drug store so I got another 27 inch iMac about $500 cheaper than it should have been :D
Anyways, as I say, apple are usually pretty good about replacing bum parts. As for Mavericks .. I like it, I can't say I see any huge performance boost over previous OS's but that may be because I've used it for a while so I've gotten used to it. I do appreciate it's networking and in order to maintain a good connection between my Mac's I upgraded all my machines to Mavericks. Also it gave my the excuse to dump Logic 9, and Pro-Tools 10.
High end iMacs are actually surprisingly powerful computers .. I can't say they'd be as powerful as a previous gen. Mac Pro but I'd bet they'd give 'em a run for their money. I'm still waiting for delivery of my brand new shiny Mac Pro (3.5ghz 6-core). But here's a nice thing I found out .. using the thunderbolt connection you can use the 27inch iMac as a display for the Mac Pro, and it works as fast and well as plugging it into a standalone 27inch cinema display .. so that's pretty cool. So I already have 2 x 27 inch screens for my Mac Pro to use. That's very nice as it saves me spending another $1,000 on screens for it.
Jusr wish it would hurry up and get here!!! ::taps foot::