My drive train was wearing out and I was looking at different ways of fixing it. I can always throw the bike at a local bike shop and pay them. But being the skeptical self I don't always trust whatever the bike shop recommends. Last summer I met a guy what works for the local outdoor equipment co-op. He builds his own bikes, has a workshop in his garage and sounds very knowledgeable. So I asked him last week if he can change the drive train for me. He said it is simple and would not take more than an hour. So I picked up the parts at his shop, cleaned my bike (now that's a BIG job by itself) and took my bike over at his place.
Once I'm at his place I realized my riding philosophy is very different from his: I ride rain or shine and he would not ride in the rain because it damages his bike too much. Partially because of the amount of gunk and partially because of my worn out stuff it took a while just to change the chain. We got the chain ring wrong and we determined that we can try keeping the rear cassette.
In hindsight it is one of those jobs that a normal cyclist would not do and it is hard for anybody who does not do this for a living to do it consistently once every few years. Add to it nuances with new technology (e.g. you should not fit a 10 speed chain on a 9 speed drive train) makes this thing a non-trivial matter - never mind what anybody tell you how easy it should be.
The next morning I was cautiously riding to bike to work. I was crossing Burrard bridge when I heard a few clicks and a big snap. Turns out my rear dérailleur snapped out from the frame. I took the bike to the shop and the guy said I have to replace the dérailleur too. So a few hundred bucks later I have a new drivetrain.