Chris, it sounds like you had the old Transpac, not the TP II, or am I wrong? The original Transpac had several faults, which have been rectified with the TP II.
I have the Transpac II with Recwing and love it. I don't have any of the problems you mentioned. For use with my single 10L x 300 bar and 3L x 300 bar pony it's ideal - very comfortable and lots of buoyancy and the D-rings are in exactly the right place for me. As regards it breaking, DiveRite did have a problem with the back-pad. Mine (and many others) broke, but they now have a new, better back-pad and replace the old one free of charge if yours breaks. For £150 I wouldn't hesitate.
You ask why they are selling after such a short time. The answer is probably that they've been told by a lot of people that they should have gone for a bp and harness instead.
It's soooooo cool and fashionable
PS Mark, if it hasn't already got one, get a crotch strap.