Why not the new Rev B MacBook Air ... it's surprisingly fast, or the 17" MBP or MacBook or a desktop/iMac/MacPro?
I could have gotten a MacBook instead of MBP, and in hindsight I probably would have saved some $$.
I wanted a laptop because I have some need for mobility, not a lot but some, and it doesn't make sense for me economically to have a desktop and a laptop. So that means I need to choose between apple's laptop line.
MBA, great looking computer, but your paying a premium for the look and its underpowered and under configured compared to the MacBook. That is I'm paying more for less.
MB/MBP 15" I struggled on this one, going back and forth between the two and the two major components the MBP has over the MB won me over, first and foremost was the GPU, giving me a discreet GPU was something that was important because I use Aperture which is quite demanding on the GPU. I found out after the fact that Aperture performance on the 9400m is great so I really didn't "need" the 9600m but its good for future use. The second thing is I read how the MBP's screen is more vibrant and less washed out then the MB, particularly at angles. I thought I'd be frustrated by this more then anything so that pushed me into the MBP. Firewire was also a consideration but not a huge draw.
The 17" wasn't really a serious contender because of the size, its just too unwieldy for my needs. When I travel, I fly most time with my young kids and trying to lug that around with the gear for my kids isn't feasible. Besides since the majority of time the laptop sits on my desk connected to a monitor, the advantage of the 17"MBP is mitigated.
All in all, I think the 15"MBP fit my needs the most, giving me a right mixture of a workable formfactor, performance and price.