I have purchased 4 engines from BGF as well. The first bike I built..I ordered a black Z280. I probably had around 600 miles on it when I sold the bike. It ran real good and was running good the day I sold it. The engine I have on my current bike that I ordered from BGF is a Z80 that is claimed to have "high quality" main bearings. Don't know if that is true or not, but with about 300 miles on it....it is running great and has power to spare. The other two engines are engines I ordered to use to build a couple bikes for friends. One friend I ordered a black X80 from LEB(same as BGF) and it is probably the smoothest engine, but is also the slowest which I beleive is do to an overly restrictive exaust. He was happy with the way it is...so I didn't mess with trying a different exaust or modifying the one he had, but it is a good runner regardless.....just not a race winner. The fourth engine I ordered was another Z280 in silver from BGF. This engine I just mounted on the bike a few weeks ago and it only has probably 25 miles on it. It seems to have a much less restrictive exaust then the X80 my other buuddy has, but it does have some clutch noise that don't sound real good...so probably going to have to dig into that one and see whats happening there. Other then that noise...it runs pretty good for an engine that is just starting its break-in and is running rich.
So my experience with BGF has been pretty positive as far as the items I have bought from him. I have never had to contact him for any type of customer service..so don't know how he is in that regards, but the 4 engines I have ordered have been as expected. I think alot of these engines are just luck of the draw. A half dozen or more different China factories build these engines and I would imagine that some are doing it better then others. My guess is that some of these engine sellers get there engines from more then one factory and the your odds of getting a good or bad engine just depend on what factories container they pull your engine kit from.
So if you are looking to get the best price possible...then you take the gamble that they pull your engine out of the right pile of scrap. So far I have been pretty lucky, but I also understand that by taking that risk...the next engine I order could be total crap. So if a guy wants to avoid taking as big of a gamble, a guy can pay a bit more to seller who claims to have inspected your engine before it gets shipped to make sure that you are recieving one that appears to be ok, but since these all come out of china...they are all sort of a gamble.
Now if you can find a seller who tears the engines down and replaces the inferior bearings with high quality bearings and inspects all the interior and its moving parts for defects...then you probably stand a good chance of getting a fine engine. However...that kind of attention to details is going to cost you and if you are mechanicly inclined, you are plenty capable of tearing down one of these engines yourself and going thru them properly before running.