In my case, the only ones I've ever built were vintage daytona's and big crown subs from the 50's.. It's very satisfying to source parts and gather them up, also to talk to other collectors and make relationships in the gen world.
The parts aren't getting any cheaper so I'm really never concerned with getting my money back, if I can't sell it to an enthusiast I can part it out quite easily.. Mixing rep and gen parts is inevitable, not every part is available or affordable in it's gen form, and the idea is to get as close as possible. It's also nice to be able to wear my frankens, I would not wear their gen counterparts, those would have to be stored away. I can't really explain it but there's something very magical about the whole process. You either love it or you don't I guess. I'm fortunate and can probably buy a genuine 6538 if I tried hard enough but I rather make my own. I will gladly pay big bucks for a modern watch that I like, but I can't say the same for vintage Rolex's. The fact that they are tool watches and were very reasonably priced when they were available to buy from Rolex makes it very hard for me to justify spending 60k for a 300$ watch. I Just can't do it.