Depends what you want to do. There are several dataloggers that read generic OBDII data, which is fine for some simple diagnostic work but rather limited for anything else such as serious tuning. If you want to be able to see and log all of the info being processed by the ECU (all sensor inputs and signal outputs) you need a Delta Dash, which you can check out at www.ecutek.com
The DD is the only unit, other than the Subaru Select Monitor used by the dealer, that can read the full set of ECU parameters.