IMO, the edge is only good to get on if you are either getting a cheaper phone and want to stay off contract, really want to stay off contract with a more expensive phone and not have to pay the large off contract price up front, or you like to upgrade phones a lot and have the money to throw around for it.
For the M8, if they did it correctly, it would be like an extra 25 dollars a month for 2 years. It's supposed to be the off contract price broken out and financed basically over a 2 year period. You can upgrade at any time however, without renewing your contract by paying for at least half the cost off the phone and turning the phone in. Meaning for the M8, you'd have to pay at least 300 dollars totaly (either by 25 dollars a month or whatever the difference is between 300 and what you've paid so far when you want to upgrade).
Doesn't sound like a very good deal to me. But then again, I still have unlimited data
If I was forced to give up my unlimited data for some reason, I might would consider the edge, but most likely I'd just buy an off contract more reasonable cost phone like a nexus line phone or something. The amount price of off contract phones is mostly ridiculous and way higher than they should be. These things could be sold for 300 - 350 dollars and the companies would still probably be making a profit, you can look at the Nexus 5 to tell that.