just bought the 2.4 5 days ago, and so far, I am impressed. previous machine was a 12" powerbook G4 and the new macbook is WAY faster. I was actually going to get the 2.0, but then decided to open some large apps in the store on the 2.0 and the 2.4 and the difference in speed was enough to sway me. that, and I dearly love the lighted keyboard. =)
I went with the 2.4 because of the backlit keyboard, but not so much that I wanted it, but that the only way my wife would let me buy one was if she got something she wanted in it and that was the backlit keyboard.
2.4 for the pure reason that it has a backlit keyboard (which I'm enjoying right now BTW). It's really just a portable machine, I don't need the 400 MHz difference, nor will I notice it, but the backlit keyboard was enough for me to justify the 2.4... besides, I had the money and the knowledge that I have the higher end machine makes me feel like the MacBook is actually faster.