Sneak Attack variant, advice needed

This is for a Rogue type class I'm working on:

Momentum: At the end of a charge, you do +1d6 damage. Improves at half the rate as sneak attack (+1d6 at 2nd level, +2d6 at 6th level, +3d6 at 10th level, et cetera.)

This will be on a class with light armor, d6 hp, and average attack progression. The class does have a number of speed bonuses and acrobatic charge. No magic, fighter bonus feats, et cetera.

Thoughts? Inherently broken? Interesting?

My thought is that its very easy to use and fairly difficult to stop, which worries me a bit. On the other hand, it does leave the character extremly vulnerable to a counter attack. And, with d6 hp, a solid counter attack could be extremly ugly for the character.

-Mei

It's basically the Skirmishing variant from Complete Adventurer, but with less powers (no AC bonus) and more restrictions (must be a charge, not just 10 ft of movement).

So, it is less powerful than the Scout's ability, so I think ti will be balanced, or even under-powered. Depends on the rest of the class really.