After owning an iMac for 5-6 years, the hard drive had filled up, and the times required to boot and open applications were very slow.
Ordered up the parts for RAM and hard drive upgrades from iFixit and followed the online instructions, referring back to the video guides as necessary. This took me essentially a full working day -- not the replacement of the physical parts, but the restoration of the software/the desktop. The steps involved in the latter were not intuitive and took lots of poking around the Internet to find what I needed. Nonetheless, I got there in the end.
As you go through any procedure in iFixit, follow instructions closely, improvise where you feel you can get away with it, and use Google frequently as you restore your system after replacing physical components.