I have not yet read any of the responses, so my concerns may have already been addressed.
There is not enough information to vote.
a) Is this feat-BAB in addition to normal BAB, or is it a replacement of normal BAB. Will every point of BAB come to you via feats, or is this a bonus on top of what is normally received every level or three? If it is the only source of BAB, then it should grant at least 1 or 2 points per feat, whereas if it is a bonus, then perhaps two feats should grant a single point.
b) Does this feat allow for multiple strikes if the point of BAB received pushes total BAB over +5, +10, etc? That would significantly up its worth.
c) +1 BAB is worth more at lower levels than higher level, IMO. At higher levels the additional strikes - especially the latter ones - have little chance of hitting. Also, at higher levels enough magic items are potentially in play that +1 to attack is not worth as much as it is at lower levels, where character stats matter more than magic items available / in use.
At lower levels, stacking with normal BAB, I would perhaps allow the use of two feats to gain +1 to BAB. The benefit from the bonus and the potential for a second strike earlier than normal is simply too great for anything less.
At mid levels, stacking with normal BAB, I would perhaps allow one feat for +1 BAB. In all likelihood, it would require the prior two mentioned feats. The benefit of getting a third attack early is not as great as it has little chance of ever hitting, and the magic in play has started to really be felt in terms of bonus to attack (from enhanced weapons, spells, etc).
At high levels, I would certainly allow +1 for one feat (although, as before, I would likely require the prior two / three feats mentioned). The benefit is now no longer worth as much. An additional attack will not matter much, as it will nearly never hit, and so many sources of +# to attack exist in items and spells, etc that the benefit's worth is further diluted.
Now, if BAB were gained only from feats, then each feat should perhaps grant +2 or +3 to BAB, although a level cap should exist for each feat: the first requiring level 1+, the second level 3+, the third level 6+, etc. That way - if this series of feats were taken every 3rd level and once at first level - the total at level 20 would be either +14 (from +2 per feat) or +21 (from +3 per feat). Hmm, it sounds like +3 per BAB feat would work best if the feat is the only source of BAB available.
Only the fighter could afford to take all of them, using its even level bonus feats for combat feats other than BAB increasing. The other classes would have to take the BAB feats occasionally, but their specific other needs would prevent their BAB from ever reaching all that high.
I still have not voted nor plan to vote, but the above is more or less my answer. For a more detailed explanation of my views on how it should work out if it stacked with normal BAB, consider the following:
feat 1 - (Pre-req feat that grants a small but otherwise minor / useless bonus in combat)
feat 2 - atk +1 feat, requires the prior and natural BAB of +1
feat 3 - atk +2 feat, does not stack with the prior but requires it and natural BAB of +6
feat 4 - atk +3 feat, does not stack with the priors but requires them and natural BAB of +12
feat 5 - atk +4 feat, does not stack with the priors but requires them and natural BAB of +18
So the first could be taken at first level, if both feats are taken, while the second could not be taken until level 6 (for fighters), the third at level 12, and the fourth at level 18.
Non-fighter (3/4 BAB) classes could take the first at first level only if human - otherwise they could not take it until level 3, having used all their feats for it. The second could not be taken until 9th level (technically 8th level, based on natural BAB, but no feat is available to most classes at that level), and the feat cost (3 of 4 feats for non-humans) is still quite high. The +3 to BAB feat would be the last they could possibly take pre-epic, and it would not be available until level 18 (16th, technically, if possible).
(1/2) BAB classes have it even worse. The first could not be taken until level 6 for non-humans, and 2 of their 3 feats would have been used for it. The +2 BAB feat would not be available until level 12 - and it would be the last pre-epic feat of this series they could take due to natural BAB limitations.
I have not decided as to whether the above should allow for additional strikes, although considering the nature of the question, I believe it should count in all ways as natural BAB except one - it does not aid in determining whether the BAB requirement is met for feats of this series (or, for that matter, any other feat requiring BAB, as a balance is presumably built into such limitations, and this balance should be maintained).
