Coming from the 8051, the PIC (12/14/16/18) is like scraping your knuckles. As for its popularity, you've got to hand it to the marketing guys at Microchip, they did their job well. I am impressed with the architecture of the PIC - it was a matter of getting the most out of the silicon that was available in its day, but I dislike programming it because you need to rely on obscure tricks. I did a project recently using a PIC16 and I'm sure I could've done in half the time on an AVR or 8051 (hc08...) as I battled with the crap interrupt mechanism. The limited stack wasn't a problem as my code is reasonably 'flat'. With the other various micros I've programmed, the 8051 wears well when used in its intended sphere of application.

With the PIC, I ended up using macros to give me the instructions I wanted and to hide the 'evil'! This makes the code easier to read (in my opinion) and easier on the programmer. Pray the code size doesn't exceed 2k!