IMO rebuilding is worth done if the mech is well versed about UDM tyranny..i have seen my mech rebuilt e39, e65 gboxes etc.he normally offers a guarantee to his customers n i believe he wouldn't want recurring issues .at least i know that he uses the new BMW parts for the gb as preventive measures. some mechanics suggest the recond unit because its easier for them ..my friend used the recond gb n it failed again after 8 months...i always believe in rebuilding the gb, will cost about the same but it gives me more peace of mind..cabuutz