Here are the main things you need to consider when buying a bluetooth headset:
1. Sound Quality: the good headsets have some kind of noise-reduction/anti-wind technology. Look out for this. If you can try it out in the shop even better.
2. Talk Time: this is the amount of time you can talk (non-stop) before the battery runs out. General times are between 6-9 hours.
3. Standby Time: amount of time your phone will last when not talking. Standard times are around 200 - 250 hours.
4. Bluetooth Version: at the moment Bluetooth Version 2.0/2.1 is the current version. It has better data transmission speed, lower power consumption and improved bit error rate than 1.1 and lower. Don't go for anything less.
5. Compatibility: always check that the bluetooth headset you are interested in is compatible with your phone/PDA/etc.
6. Size: this will be a personal choice, however it usually determines battery times, i.e. smaller the headset the shorter the talk-time and standby-time.
7. Support: whether you're a novice or an advanced user, it's always handy to be able to call the supplier to get any issues resolved.