yes they work well. ive gone through 3 since the warm a15 went in.(some year and a few months ago) jeff taylor is right. the other way to destroy them is to launch (off lights or around 90degree bends) all the time and not stop when 2nd gear starts whining
the 120Y or sunny 56a box's are definitely the best (the younger the better, better than a 30yr old 56).
you can drive around forever in a 56a with a warm a15 as long as u dont ever put the foot to the floor! thats when the torque of the motor hits the gears and wears them!

edit : re the rebuild, i rekon even rebuilding a worn old 56a box is a waste of money, when they break i buy a new-old one (from wrecker) and trade in the old ($200 a pop). a rebuild usually isnt new gears, just synchros and seals and stuff. the gears remain worn and old. it would be sweet if u could just rebuild one and not have a problem