i have managed to buy half decent bikes all the way, although have sold most of them for one reason or another. i suppose the worst was a Norco vfr4 disc, which was a nice enough bike in itself but there was a fault with the fork dropouts so i returned it after a week and got a mountain bike instead. lucky escape as really the Norco was too similar to my then main bike to justify having it.
worst bike purchase i know of, and the most infuriating, was when my sister, despite being well aware of my addiction to bikes and bike shopping, chose not to involve me at all when considering her first bike, and then rang me proudly to boast of having bought a muddyfox for £130 at costco. it lasted about four months before bits started falling off. i made sure she took me with her next time.