Well, I was given a 2009 Macbook Pro at work, and for the longest time, that's the only computer I had. However, about a year a go, I realized I wanted my own desktop, and got a good deal ($1,350) on a 2006 quad-core MacPro. It already came with 7Gb of RAM, and 1.5Tb of disk, so, I just added 4TB ($250), an ATI 1800XT graphics card ($150), and two Dell 3007WFP 30" monitors ($950 for both), and I'm as happy as can be. I've thought about upgrading at some point to a newer one... but I honestly haven't found any reason to do that.
My day job is software development under Eclipse, and for that, the 2006 MP is roughly 2x faster than my 2009 MBP. If I get an SSD for the MP, most "slow" stuff will be significantly faster, since dev stuff is mostly bound by IO (loading huge amount of files into memory, writing compiled code back to disk, etc). So, spending $150 on that, should allow me to hold off on upgrading until at least 2012. At that point, I may just buy a gently used 12-core 2010 MacPro
The MBP is very useful when I'm traveling, but when I'm just around town, I very rarely use it. Most of the time, my phone is enough for a quick search. If travel was <10% of my time, I would definitely go for a nice MP rig at work too.