they are very similar to the 700r4 used in the thirdgens. most of the same components were used and only altered to allow for teh elctronic shift control. they can be built to be very very strong, 600+ft/lbs can be put on teh input shaft if properly prepared.
the two weakest links in the tranny staock are teh same as in teh 700, teh sunshell is a POS and the 3rd gear accumulator is a little less beefy than needed for performance. both are easily modded/replaced in teh aftermarket, so no big deal.
for parts i suggest
http://www.transmissioncenter.net/
they have a killer line of components and complete units all at reasonable prices
later
tim