I'm not big into most extended warranties, but I usually buy Applecare. Most of the Apple machines I've owned over the years got very heavy use and something always went wrong down the line outside of the 1-year factory warranty...
1. 2 laptops: 15" 1GHz TiBook (SuperDrive replaced TWICE, back when they were $300-400 parts) and a 12" 1.5GHz PBG4 (SuperDrive replaced once, logic board replaced after two years). My 15" 2.6GHz MBP has had a couple of in-warranty repairs so far - new logic board (faulty NVIDIA 8600M chip), new hard drive, new SuperDrive. Now, that I think of it, every Apple laptop I've owned eventually needed a SuperDrive replacement...
2. PMG5 dual 2.5GHz: leaking CPU heatsinks 2.5 years into ownership. Received a new logic board and new processors. Total cost of parts: around $1,800, I think, if I had to pay for it.
3. iMac C2D 2.16GHz: that one needed a new SuperDrive too, about 2 years in. No other problems though.
So, I think I got my money's worth on Applecare over the years. I bought it on my 2008 Mac Pro as well.
The other way (unrelated) that extended warranties have worked in my favor are with game consoles. They stuff so much powerful hardware into tiny little boxes that something always seems to go wrong. My XBox 360 has been replaced TWICE (one for dead disc drive and another for the red ring of death), my PS3 once (dead disc drive) - both outside of their factory warranties. But never a problem with my Wii, strangely.
PS. User replaceable is 1 thing. The price of parts is another. It's great if you know how to replace the motherboard yourself, but it wouldn't change the fact that it might set you back $400.
Hah. If only Apple logic boards cost "only" $400... The typical PMG5 or Mac Pro logic board hovers around $800+.