I like what they do to me. I also recommend them to everybody I train.
the lower/upper back, hips, hamstring workout is insane!
your, believe or not, ABS will get good workout too!
It is true that it becomes really akward in your arms with anything over 100 kg (I think it is 220 lb), but I sure it's well worth it!
I used to do sets @150 kg (333 lb) with small towel only and survived . . . hance it can not be soooo bad on your arms
P.S. Try the same thing with sand bag - really good shit!!!