I chose the Nike Imara HRM after EXTENSIVELY researching several user reviews and product descriptions/comparison charts on various websites:
I looked at Suunto HRMs b/c I use their dive equipment, but most user reviews were unhappy with the materials used and it is not water resistant like Nike's HRMs. Similarly, the other lines do not offer the versatility of Nike.
I was then torn between Nike's Imara and C6 (this site doesn't sell C6). I settled on the Imara mainly b/c of size and it's features. I am VERY happy with my purchase. It works well: it's simple to use and the charts in the manual have the same recommendations as my trainer (who also uses the old Imara). It takes a couple uses to get used to how to use it optimally with and without the zone alarm (which may bug others around you if it is turned on b/c it beeps every time you go out of your zone). You just figure out which way works best for you and set it to the right mode each use (I use different modes depending on what and where I'm working out). My trainer suggested sleeping one night with it on before training with it so that upon waking you could see your true resting heart rate.
I went back to the comparison board before doing this review to look again at the C6. They are very similar from what I can tell...still would choose the Imara.
I purchased from this site simply because they back up their products with a return policy...but I don't need to return it. Great service! Fast and reliable.