I've built 4 so far. It's funny cause years ago I got into pedal building then got into amp building and now I'm back to pedals again. I mean, I can build about 100 pedals for the price of building 1 tube amp and it's way quicker. I started out with a Marshall 18watt kit from Weber and then went on to sourcing my own parts and all that and built a Princeton, tweed Bandmaster, Lightning 15, and a 100 watt Super Lead with the Dave Friedman brown eye mods. Really fun and if it's something you'd be interested in check out www.ampgarage.com Crazy knowledgeable people on there that have always been very helpful to me in the past. Also check out
http://www.valvewizard.co.uk/ for a good basic understanding of how tubes work.
Tube amps are obviously more complicated and there's a good amount of danger but really rewarding! And nothing feels better than getting a compliment after a show on how good your gear sounds and you can reply,"Yeah I build it all."