"Light" doesn't automatically mean "good", especially when it's at the expense of efficiency, durability or comfort.
I could not agree more, I used to have a sub 20lb custom Ti hardtail and it was by far the worst bike I owned. I sold it on pretty quickly as whilst it was the fastest bike I owned it wasn't a fun ride.
It all depends what kind of riding you do, racing vs riding with your mates:
I use to think that being a sub 140lb rider that I would notice the difference between a light bike and a heavy bike more than most riders, and the truth is I do. A few years back when I had the aforementioned Ti bike I spent a summer recording my times round the local trails on various bikes, and yes the light weight Ti bike constantly recorded the fastest times, and surprise surprise my Orange Five AM set the slowest times, however the Five was a lot more comfortable and fun to ride. If I was to have gone into racing it would have been a no brainier as to which bike to use. These day's I've stopped riding on my own and ride with the misses a lot more, whilst she is marginally quicker than me down hill I find winching back up a lot easier (and quicker) as such I no-longer benefit from having a light "race" bike but I do benefit from having something that's fun to ride. So whilst I try not to carry nu-nessessary weight but at the same time I'd rather spend £100 on a nice evening out than a set of Ti bolts that save a few grams.
As I'm not a competitive racer I don't need to keep secrets so here's my tip if you want a faster bike: Rather than spend vast sums of money chasing lighter & lighter bikes invest in the best bearings you can (and the truest wheels you can get) until you experience it you won't believe the satisfaction you get from coasting past someone who's spinning out.