I haven't tried to properly max this out yet, but I did already have a rogue who got a couple of extra charges due to the enhancements they were taking anyway. I tried taking advantage of that and my feedback so far is:
Summary:
Overall, I think it's an excellent concept. The speedy movement speed was cool and all, but I really like the core concept of it being a 'tactical use' ability.
But I think just needs to be a little more permissive to make it a more generally "fun", and the scope should be broadened to think of more interesting reasons for *non* tumble monkeys to want to use the five foot step charges that they have beyond the dodge which thematically doesn't really work for me in terms of what you do whilst *blocking* if you're not actually built (skills & gear wise) to 'tumble'.
More detail:
First: Icons are too small at max zoom out (which is how I like to play), it looks to me like they're "attached" to the character rather than being a UI element. Because of this, if you *do* want to make a character that is meant to properly use this mechanic, maxing out all the relevant things... its very hard to keep track of what you have left in the tank so to speak.
Second: +1 charges as a baseline for everyone, more classes getting ways in their enhancement trees (or racial trees, or universal trees, but mutual-exclusive I would suggest) to boost their max charges. Getting immobilised whilst blocking is boring, it needs to take just a little longer for that to happen please.
Third: Tighter integration with the actual tumble skill, e.g. +1 charge based on actual Tumble skill break points, or faster recharge time at those break points. Or both.
Fourth: Introduce like a "reverse runearm" thing in the mechanic: the fewer charges you have when you use a charge, the less effective the tumble is? i.e. as you start to run out you *start* to get tired and less effective, but you can still keep going for now. This will encourage people to *manage* their charges tactically based on the opposition / difficulty level, rather than just trying to avoid running out of them.
Fifth: I think if the recharge rate is sped up (or you can build/skill for it) in some fashion as per suggestions above, then the first charge from zero should be set to recharge much slower than the rest. This will maintain the incentive *not* to run out whilst allowing you to still make "frequent" use of charges. i.e. when you actually run out you are now properly *tired*, and really can't keep pushing yourself anymore without a little bit of a break from all that.