I've used/been using PIC (not for 6 years) AVR, LPC21, LPC23, LPC17, LPC13 chips. And nowadays CC430, STR32F0 and STR32F4.. However, for 2 years I've been using Cortex M3 and M0's for most of the time. CC430 series for it's integrated wireless.. For different projects, different chips offer different peripherals and different prices.. I also help people on their migration to ARM processors..
What I see is, ARM chips have a lot to configure to do simple tasks. IDE's are garbage and frustrating for newcomers. CMSIS, is really ugly code to deal with.. All other Keil's library are garbage.. Even the IDE's are not just plug and play. If you download Code Red's IDE, you just can't get flash a led easyly.. ARM is a little difficult to grasp for people.. Just think of GPIO on AVR/PIC and in STR32F4 series.. Look at the number of pages you have to read to work on.. AHB clocks, Mode registers, SET/CLEAR registers, alternate function registers.. You have to do a lot of bit shift, and, or, not's.. It is really difficult for a starter.. And look at an AVR. Just a few easy to understand registers, and you're ready..
Another example is the timers.. LPC series have really bad designed 16/32 bit timers, however they are still powerfull and more complex than PICS and AVR's. STR32's??? They are really but really nice, loved them, but not for starters.. What about UARTS, SPI's, I2C's..
People just hate reading manuals, and big companies are good at creating junk..
The common problem across the new starters of LPC is the IAP pin.. You have to pull up the P2.10 in reset to let MCU run the user code.. This information is somewhere in the manual, not obvious to the one who just jumped in.. etc. etc.
To sum up, ARM is difficult for starters, there are a lot of walls the starter is going to hit until just run his first code.. However, if you get it going, you won't look back to 8 or 16 bits, unless they offer something different. For instance some PIC's have CLCs. If you need a few and's and OR's and CPLD's are a little big for your application, why not use 10F32 instead of descrete logic chips?
BTW, the worst chip and documentation I have ever had is MSP430 series. (I know it is not ARM) It's hardware is nice. Having DMA on a small chip is fantastic. The pleasure of being able to connect any pin to any peripheral cannot be described.. But API and documentation is really really bad. Even there isn't a naming consistency in the macros. What I see is big companies have the ability to create hardware but not software..