They an run up to 8Mhz (up to 16 Mhz for the new MSP430F2xxx series) and their consumption is only 250 uA per MIPS, which makes them perfect for portable and handheld devices.
Very low power: 250 uA per MIPS.
Mature product with very clean architecture - code porting between different members of MSP430 family needs alsmost no re-touch.
Avaliable in any quantity through TI distributors, with lots of application notes and code examples written.
Very good range of peripherials: Precise ADC and DACs, Comparators, OAMPs on chip, LCD drivers, SPI, UART, DMA.
The new members MSP430X have extended instruction set and allow more than 64KB memory addressing.
Von Neuman architecture, instructions take from 1 to 4 bytes, and are performed in 1 to 6 clocks.