I've lost two batteries to overheating: both oozed some kind of black stuff. Both were damaged when given a full charge which heated the batteries excessively. Topping off a partially charged cell doesn't allow the heat to build up. This is kind of stupid, but a fully discharged battery should probably be charged in two stages, allowing the battery to cool.
Despite that I still like the system. Ray-o-vac sent me four extra cells, gratis, so I'm still two cells ahead. I still get fast charges - now that it's winter maybe the heat won't be such a problem. Also I'm willing to sacrifice cell longevity for the convenience of a fast charge.