I have a Polar F7. It works well for me, reliable and easy to use. It gives me my heart rate and what it thinks are the calories I burn. Slightly better then what gym machines say but IMO disregard the calorie valuation, they are grossly over estimated in any machine and also take into consideration your BMR which means your not really finding out how much more your burning, your finding out your total calorie burn, eg including that which you'd burn if you were on bedrest all day.
Garmain seem to be the best models for running outside as they give you GPS details. People who have Garmains seem to love them but they come with a heafty price tag. IMO if your new to the whole HRM thing you'd be better off starting with a basic model and working out how you can better the model, thinking of functions you do and don't use or want.
Polar do many different models and it depends on what your needs are, what your aims are and what your price range is. If you go to their site you can take a quiz which advises on which models would best suit you.