Have used it as written and liked it.
It's not just Improved Brawl. It's Improved Brawl, Streetfighting, and Power Attack. You get those -- all of which are Tough Hero bonus feats and all of which except Streetfighting are Strong Hero bonus feats -- and you're golden.
The Tough Hero at 4th level, with his two bonus feats and two normal levelling feats, could be doing 1d8+1d4+Power Attack +Strength, and be getting a +2 to hit in the process. A Strong Hero at 4th level, with his two bonus feats and only one levelling feat, could be doing 1d8+Power Attack+Strength+Improved Melee Smash, also with a +2 to hit. And if you've got a feat slot free, give him Streetfighting as well, and you're doing, what, 1d8+1d4+2 (Melee Smash) +2 (Assume a 14 or 15 strength, not even forcing a 16) + Power Attack damage, and average of 11 plus whatever you're power attacking with. The Tough guy does a bit less damage, since he doesn't have Melee Smash, but he's got more hit points and some nice stay-alive talents to make up for it.
For me, the Power Attack is the key. That's what (I think) the rationale for giving you the bonuus to unarmed attacks is for -- it's assuming that you'll be Power Attacking to get your damage up there high enough to force a massive damage check.
And what I think some people forget is that you're not SUPPOSED to force a save every time you hit, even with a character focused on that. If you even force the save, you automatically make them lose their next turn, which is just a killer in d20 Modern fights. If you can make somebody lose their turn one round out of three, you are seriously debilitating them, even if they keep making that save. I don't think of it as "Chance to get a knockout, assuming I hit, I roll well on damage, and they fail their save." I think of it as "Chance to make them lose a turn, assuming I hit and roll well -- and I could one-shot them right out of the fight." In my mind, that's a significantly better way of thinking about it.
I've heard (but am not saying this is widespread) that some people think that the automatic-daze feature is too strong, and don't use that rule... and then complain that non-accruing damage is weak.