The teensy is a great arduino compatible board, but I wouldn't recommend it for beginners, because some of the arduino libraries won't work without modification. And of course it is not shield compatible.
It has some advantages over the standard arduino. It can function as a USB HID device- it can act as a USB mouse or keyboard without additional hardware, and without needing any drivers installed on the PC. And the Teensy++ has memory and I/O similar to the arduino mega at less than half the price.
-transfinite