The practical benchmarks that matter to me (Games, AntiVirus Scan, WinRAR), the Phenoms are pretty much equivalent to the same-clocked Core 2 Duos.
When it comes to multi-threaded applications (Supreme Command PC game, WinRAR), the 2.4GHz Phenom (4 cores) is inline with the 2.66GHz E6750 (2 cores). I imagine this is because of the two extra cores.
The Phenoms will have less power consumption during idle (most of the time my computer is in use). But it has a higher TDP than Core 2 Duos, which suggests they draw more power when all 4 cores are loaded 100%.
The Phenom, for my purposes, will only be a disappointment if they are priced above or at the same price as equivalently clocked Core 2 Duo's. In other words: they better be cheaper than Intel's equivalent offerings or else I'm not interested.
In the case of the 2.4GHz Phenom, it will be a disappointment to me if it's priced above US$180-190 retail, which is what the 2.66GHz Core 2 Duo is going for now, since 2.4GHz Phenom = 2.66GHz Core 2 Duo for the applications I am interested in.