The mantle is a great tool. Catch someone when they are concentrating on another ally and it's an easy kill. But it's very situational and the counter does make it more risk/reward than it ever was in Gears of War 3.
Which is a good thing.
Not sure if people would agree with me but I'd like to see the Yank and Shank sped up a little. It's too slow and leaves you completely open - which is again, good as it's more risk/reward.
But, as it stands, no one has the chance to use it. Most people meet another player in cover shortly before an 'Up-A' shotgun rush, no one bothers with it at the moment because it's not worth it.
That's not to say a change wouldn't ruin it, but I'd like to see them toy with it a little during the beta.
Edit: It's funny to think the former mechanic completely nullifies the latter mechanic. One is designed to keep you moving and on the offensive, the other is a punishment for getting caught out for not moving.
The new mantle means, ironically, you're less likely to take cover in front of anyone.
I suppose it plays into mind games - 'I'll wait for the vault' BAM!