I'm a big LEGO nerd and have 8 or 9 of these (including the smaller, more shallow version) to sort and store LEGO. The smaller ones stack perfectly with these larger ones, they just aren't as thick.
As has been pointed out, these are really high quality, stack well, and while you can't adjust them the way you can the other kinds of parts holders with the dividers, they hold the pieces better without them eventually working their way into the other compartments, the removable bins make working on projects a lot easier - grab the bin you want, dump it out if you have to, or root around in it looking for the specific piece you need (in my case it might be a specific color).
Having tried ones with the dividers, I can say that, for me, these are much better.
EDIT 8/21/2014: I'm back for more. I'm completely resorting my LEGO collection and buying more of these (and the smaller 25 bin ones) - after all these years, I still think these are the best way of sorting and storing LEGO.