>>41
The MSP430 is supposed to be OK, haven't used them myself. TI are trying to cash in on the Arduino phenomenon with their value kit, but it's not nearly as popular which also means it's not as easy to find someone to help you if you run into problems. You'll also miss out on all the existing Arduino shields. However the kit is so ridiculously cheap you'd almost need a reason
not to get it. As your skills increase you'll find that switching between MCU architectures is less and less of an issue, so just having a spare board you can dig out of the closet for some quick project can be a great timesaver. You might want to check that the tools run on your platform of choice though. GCC supports the MSP430, but I don't know if you need some special Windows-only upload tool to program the chip.
The PICAXE is in the end just a PIC microcontroller, as long as you have a way of getting your code into the chip there's no requirement to use their environment. Similarly just because you're using an Arduino board there's no requirement that you have to use the Arduino environment or libraries. For one project I needed an AVR board and an Arduino Mini was the quickest way to get going. The first thing I did was to erase the Arduino bootloader, and just used avr-libc and the built-in flash programming interface.