I’d venture my arm that once you exclude supermarket specials/BSO’s/whatever then you have a messy pile along the lines of how much did you pay as a function of what you could afford and what your expectations were multiplied by how much fun you had on it.
I’m pretty sure that I wouldn’t automatically have more fun on a £6000 bike than on one of my bikes – all of which feature second hand parts somewhere. There’s bound to be people that can get the last ounce of perfomance out of such a machine, but I couldn’t. The sheer price would possibly even intimidate me.
That said, If I blew that £6000 on a bike expecting it to be totally epic and found that my five year old Zesty was as much fun, then I might feel a bit cheated. Likewise, if something broke in reasonable use and I got bounced for warranty or any other support I’d be a bit peeved. I’d be more inclined to overlook any failings of say, a couple of hundred quid’s worth of lowend Spesh with an XC30 or whatever as it’s not a fair comparison to anything I have, although I would probably not buy such a bike new.
I’m not sure there is such a thing as a bad bike once you’re at a proper bike shop, just over hyped expectations and bad experiences.