AVR v PIC debates are always good :-)

A PIC system similar to an Uno is a comparable price, and when I looked last year, PIC compilers were not exactly user friendly.
Each have their own strengths and weaknesses, but for simplicity Arduino does currently have quite a lead, and the standard layout with lots of 'sheilds' (aka stackable expansion boards) makes prototyping far easier.

I use Mega's, Uno's and Pro Mini's. Uno's I've currently got 3 off as I use them for general development. One of my current projects will be getting shrunk from an Uno and breadboard to a custom PCB running an Atmega328 with the UNO bootloader. Final cost is very similar to using a PIC, but I get the benefit of using the Arduino programming.
The Mega's are used for a similar, but more complex system. This could be done far cheaper with a custom PCB and an Atmega somewhere between the 2550 and 328, but I only need 3, so using the Mega eliminates dealing with a chip not officially supported by Arduino.
And the Pro Mini's are for a display controller, which i only need 2 off, so other options work out more expensive. The Pro-Mini sits neatly inside the housing, with the display and serial connector wires connected straight to it, without needing any protoboards.