I suppose this is the iFixit guide you're referring to.
I have lots of experience opening/putting together electronic devices, but trying to replace a worn out CCFL backlight in my Powerbook G4's screen was a rather unpleasant experience as I wrecked a perfectly good LCD display. Then again this involved actually opening up the LCD component itself! How stupid not to make those backlight easily replaceable, but then again they'd miss selling a replacement LCD (that's green thinking for you!).
I assume inserting a 2nd drive in the iMac is nothing as risky as that though.
Is it a challenge to perform the "surgery" in a way that won't leave any traces of "tampering" in case of warranty issues?
I've been following user reports at MacRumors by people who received the 21.5 inch iMac with SSD, versus those who installed their own OCZ Vertex / Intel / other branded SSD. Apple's Toshiba SATA-2 SSD benchmarks considerably lower than most third party SATA-3 SSDs that have aggressive garbage collection. Whether such differences are perceptible to an average user may be another matter. MacRumors has all the info to step you through the installation process if you're feeling brave.
I agree - where your data is should be important, and comparing SSD and hard drive PC performance is problematic without that information. There are two other factors that affect the value of an SSD on the iMac: 1) The newest SSDs are SATA III, but the Apple bus is SATA II, so even if you bake your own you won't be able to get optimum performance; 2) The MacBook Air bypasses the SATA issue altogether by mounting the SSD directly on the motherboard using the much faster PCI Express bus; the iMac won't truly rock until it does the same.
That said, the real barrier to using SSDs is still price. It remains a high-end option. There's also the fact that SSD development is advancing rapidly so that whatever investment you make now will soon be outdated. As a result, I think an internal SSD in an iMac is a dubious investment. In my opinion one would do better to wait till an external Thunderbolt SSD drive option is available. This will offer both flexibility and upgradeability. Thunderbolt offers PCI Express speeds that simply cannot be matched by an internal SATA SSD, particularly on the SATA II bus currently available in the iMac.