I haven't tried ML yet. My MBP is brand new (bought in January 2012), factory configuration (4GB RAM, Lion)
If you use it 'lightly' (that is, only Safari open) it's a breeze. But of course, it's never only that if you want to do any work.
Frankly, 4 GB should be enough! My past machine (with 3GB - and Linux) would rarely swap (in fact I could keep swap off and use a Windows 7 VM) But you can go only so far with an aging CPU
Some of the slowness can be attributed to Safari/Firefox, sure
But it really seems to have something wrong. Maybe they really neglected people with spinning disks.
(Yes, I considered buying a MacBook Air but 128Gb was not enough for me and the other options were above my budget)