The Arduino Pro Mini is SparkFun's minimal design approach to Arduino. It's essentially a 3.3v Arduino that runs an 8 MHz bootloader on a super small, super thin board. There are no connectors attached - allowing you to add whatever connectors or wire you want without too much trouble.
The goal of the Pro Mini isn't to replace the Arduino so much as bring the cost down for more advanced users. The Arduino Uno is still the inimitable board but if you're comfortable working within voltage limitations, don't mind off-board USB, and want to customize your connectors, the Arduino Pro Mini's a great budget purchase. You will need an FTDI friend to program your Pro Mini.
Note that the FTDI port on the Pro Mini 3.3V expects both 3.3V power and data/logic! You will need to adapt the FTDI programming interface to make sure it is both 3V logic and power.