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Quite doable as a hobby these days especially if you like music. You can buy parts direct from China extremely cheap on eBay/etc. I have been (extremely slowly) building a MIDI keyboard out of an Arduino and have built guitar pedals in the past.
Also enjoy this guy's videos when I have crap loads of free time:
https://www.youtube.com/watch?v=yxtXMC7suuQ&index=2&list=PLjzGSu1yGFjXKZ5igKxwlgfGdy25yZoPN
&v=yxtXMC7suuQHe is building an analogue vocoder. Also watching him opening post is very satisfying and helps control my urges to buy crap.
Edit: fair warning, the videos aren't exactly exciting
Excellent link, thanks! Will read up a bit before buying anything.