The BASIC Stamp line is a good inroads into microcontrollers, and you can do plenty with them. I find them to be fairly expensive for what you get, though.
I transitioned from the Parallax products to PIC micros several years back, and am now in the process of swinging to AVR micros instead. There are whole families of microcontrollers that outperform the BASIC Stamps for much lower prices ($5 a pop in single-unit quantities), and it's possible to pick and choose from scaled-down micros that include just the features you want at correspondingly lower price points and current consumption.
It's great building simple machines using little 8-pin low-power microcontrollers that you can throw around like tic-tacs though - and even in small quantities, their price is amazingly low (less than $2 unit price).
The other day, I was marveling once again at the fact that I could purchase a tube of chips, each one more than 20 times faster than my first computer, with over 30 times the memory, and far more capable with built-in support for several different communication protocols, multi-channel analog-to-digital conversion, and many other features and functions that were never even approximated in that old machine -- all for less than $5 each. I think we paid somewhere in the neighborhood of $1,000 for that computer in 1978.
Admittedly, the Stamp series is very easy to get into, and I've made many projects based on the platform. I just prefer to use my micros bare, instead of packaged as Stamps. There are advantages and disadvantages to each approach. I just find price to be a pretty big motivator, especially in my current situation.