This is always best approaching it from the other end..... what do you want it to do
I use an original Garmin Fenix or a Suunto Ambit
The former can link to your phone, and take messages whereas the Ambit won't
The Garmin has a very simple map - THe Ambit doesn't
Both will link with HR or cadence, but have some limits as to teh specs
Therein lies the issue.
As above, the best comparison is that it is like using a basic GPS like the 200 in a smaller wrist mounted format.
There is a site called
DC Rainmaker who does some excellent reviews and comparisons