It's not really a 'micro-controller' at all, it's a complete system based on a PIC with an in-built BASIC interpreter, and programmes stored in external EEPROM.
I think what Nigel is trying to hint at (even though it's pic based) is that you shouldn't use them. I outgrew my first stamp 2 days after I bought it, never bought one again. When he says slow, he's not kidding, they're extrmeley slow, any PIC or AVR base micro controller module that can be programmed in a basic like language is recommended over a Stamp.