Only once out of about 20 bikes did i ever buy an absolute dog and that's just because I WANTED a CBX1000 and stupidly bought the first one i found up for sale, other than that I'v been lucky enought to never have much more than a dodgy coil (VT750) or some really cruddy and never cleaned brakes (most of them).
These days I just buy a new bike every 2 or 3 years and trade it in for the next one though if my body lets me keep going, the Triumph Bobber i picked up this summer is a keeper, it is really just... Fun