I'm typing this on a 5-ish year old Dell Latitude. I bought it from a local repair shop for $150. I upgraded the processor (to a core 2 duo) and memory to 4 GB of memory, all for less than $100.
I'm using it to run 5 different operating systems (primary system is Linux, secondary is Windows 8 and I've got 3 others). I'm a computer instructor at a local community college and I'm using it for quite a bit. Everything from demonstrating many partitions on a single disk, analyzing network traffic, playing lots of videos, virtualization, Android development, other software development, some light image editing... I have yet to find anything that really stresses it out.
Personally, I'd never buy a new laptop again. I figure I'll always be able to score units like the one I'm using now. In fact, I'll use this one until it dies.