I do all of my own maintenance and repair work. In fact I enjoy working on bikes almost as much as riding. I have learned by doing, although I (almost) always read the instructions first. I have also learned by hanging out with other people who like to work on bikes. Over the years I have acquired enough tools, manuals, and experience to be able to handle almost everything. It may take some back-and-forthing, and I may not get things done as quickly and efficiently as a shop mechanic, but it's more enjoyable for me to do it than to pay someone else. I do my own wheel truing, and I have built my own wheels (made a good excuse to get a truing stand) but now it seems like you can buy a set of wheels for less than the cost of the parts.

A lot of my bikes I have bought as framesets, new or used, and put together myself, although I am glad to leave the frame prep. (facing, reaming, and chasing threads) to the pros. It's fun switching parts around and trying different permutations. I have an old Bianchi that has been though at least three distinct incarnations since I bought it used; a 10-speed (anybody remember 5-speed freewheels?) with a hodge-podge of parts (Campy derailleurs, SunTour cranks, Universal brakes); as a loaded touring bike with an Avocet triple (the frame has a long wheelbase and eyelets on the dropouts); and its current form, as a fixed-gear (it's old enough to not have braze-ons for the cable guides, shifters and front derailleur).

While I agree that you don't always have to have the specialized tool to get something done, the headset press discussion being one example, a lot of times haiving the correct tool does make the process much easier. Whether it's worth it is an individiual decision. As a confessed tool junkie I have regarded each new problem as a rationale (or an excuse) to buy the needed tools. I got started by working on my first 10-speed, a mid-70's Atala with a Campy three-arm steel crank that kept working loose. I bought a crank extractor and the rest is history.