I bought 2 kits for a friend and me from Handhelditems and there have been little to no problems at all. The only problems I had were leaky fuel valve (fixed with some teflon tape), the exhausts needed to be bent to allow free pedal travel, and one of the kits had a problem with the carb that went away after I took it apart and resembled it. I have no hands on experience with engines and few facilities at me disposal. My dads wood shop had all the tools I needed. After about 10 hours of work we were finished with both bikes. They have driven over 100 miles each with no problems.
If I left the bikes as is I'm sure they would continue to run fine, but like many have said already, half the fun is working on these things. So many upgrades are still to come.