IMHO the only 2 negatives in fitting 1 (or 2) of these is they can be noisy on the road when doing slow tight turns, when they can go bang quite loudly from time to time and the fact that its always locked off road when sometimes having an open diff would be an advantage.
But then their massively cheaper than air lockers and probably more reliable as in you can fit and forget about them and not suffer from air leaks which can obviously stop air lockers from working. You can also go off road and not have to think, shall I put the lockers in or not as their always in.
If I was to do my build again I wouldn't fit any of the lockers mentioned in this thread, I'd be going for these, as I'm sick of my ARB air lockers leaking air.
http://www.terraintamer.com/en/4wd-products/elocker