It's not so much about how often you are going to use the reader but what for.
If you are just curious about which code you have and don't need/want any further information, a basic version will be fine. E.g.
Amazon.com: Autel MaxiScan MS300 CAN Diagnostic Scan Tool for OBDII Vehicles: Automotive
If you want to dig deeper and need detailed life data (as you drive the car) for diagnostic purposes or tuning purposes or just to satisfy your curiosity, you will need a higher-end model. There are versions that connect to an iPhone/iPod Touch or a laptop. $100-200 would get you a decent hardware/software combination.
Another interesting application is a reader that you keep connected to your car and that shows you mileage for optimizing driving habits. E.g., ScanGauge:
Amazon.com: ScanGauge II Ultra Compact 3-in-1 Automotive Computer with Customizable Real-Time Fuel Economy Digital Gauges: Automotive
