It's pretty underwhelming. I mean, as a talent that replaces an ability it needs to do one of three things:
1: Make said ability just better(like MSR)
2: Give an equivalent of said ability+something else.
3: Give something completely different that's awesome.
In this case, Siegebreaker replaces intim with a 10yard knock-back and gives something else(the 300% burst damage). So you replace intim shout for a 10yard, single target, 45 sec CD knock back. It's pretty underwhelming. For some reason I though it was going to be a knock-back followed be a knock-down... But what actually happens is they just fly through the air 10 yards and that's it. It's just a knock-back.
I still enjoy it in solo BGs/play.. but it was obviously very, very underwhelming in arenas. The only time I could see you using it is on BE vs some certain comp... Other than that, it's a pretty terrible ability.
Personally, I think it needs to be AoE. Not the damage, but the knock-back should hit the main target+others within x yards or something. And the CC part of the effect should last at least 2sec.