Because of their all-in-one design, iMacs are the least reliable of all the Mac computers, but I have a friend with a 2007 that he still uses daily as a graphic professional. The DVD drive died a few years ago, but other than that, he hasn't had any significant problems with it.
Least reliable, or hardest to do a DIY repair? I call "bull" on your "least reliable" claim unless you have some stats to back it up.
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I would be really interested to hear more about your experience with your fusion drive. Do you think it was worth doing? I have a 2008 24" so I don't think I can do the same as the DVD drive connection is limiting factor in terms of speed. I have however been considering swapping out the HDD for a SSD and just using FW800 external drives for media storage and such.
I have a 2008, and I swapped my internal HDD with an SSD, and then fused my 240GB internal SSD with a 1TB external FW800 HDD. Works great - just helping my to buy time before I eventually invest into a souped-up Mini.