Not to take away from git bisect (or hg bisect, which is no more broken than the rest of mercurial) -- I don't think it is an alternative to the thing you used to do.
git bisect is "temporal bisection", i.e. you delete half of a time interval. The manual bisect is kinda-sorta "spatial" in the sense that you delete half a region of code. the two things work by similar principles, and their problem domains overlap -- but they are not the same.
They're complimentary techniques. One might git bisect down to the commit level, then manually bisect the sub-changes in the commit if it's a large one.
I've done the manual equivalent in perforce as well.
