I'm a pretty firm believer in 'It's the operator, not the machine' ... with provisions:
1. Does your machine fit you properly?
2. How is the frame weight/strength/stiffness compared to the bike you have your eye on? Everything else is changeable... Frames are forever.
3. Is the frame still in good condition?

BC has been with me for about 3 years now... but a fully outfitted hardtail at 28# isn't too bad. Her frame's in good shape. The money I'd spend on a new bike will instead go to a new wheelset, disc brakes, avid Full Metal Jackets (avid mech brakes) and, when necessary, derailleurs, chains, etc as needed.

...but I'm embarrassingly attached to my bikey. I think I'd shrivel into a ball and cry if I lost her.