I've fiddled with the Philips lpc2103/2119 cpus. The Jtag box came with the Keil 2103 EV board. All seems to work how you would expect although I rarely use the debug The peripherals and interrupts take a little getting used to and they can have little quirks - like on the timers you can only get the overflow/capture/compare status if you've got the interrupts turned on. Read the doc and errata CAREFULLY.

At the moment I've go a Luminary Micro EV-LM3S811 ev board. My major gripe about these parts is that the peripherals seem like they're been designed by a chip designer - they have weirdnesses like no 9 bit mode for the uarts, no prescaler for the 16bit timer capture mode and you can't read the timer in this mode. This seems to be the case with many of the core based processor designs - designed by someone who is good at vhdl but has little concept of how people actually use the peripherals. The interrupts on the CortexM3 cpu are similar to how they work on the 8051 - simple,but effective whereas the other ARMS have a VIC and are a bit more sophisticated.

Keil seems to be my first choice for tools - I was able to get things up and running faster as compared with the IAR tools. The Keil front end with GCC compiler works well.

also expect the ARMs to be a bit more hungrier on rom space - at a guess around 50% compared with 8051 - this is a 'finger in the air' approximation - your mileage may vary.

Apart from a few little quirks, these devices are fast and interfacing to things like LCDs need some delays.

All in all, the 8051 ain't dead yet. The 8bit micros still can be simpler and lower power in some cases.