I'm also considering this lens to replace my 135-400mm sigma. mainly to get the OS. Figure if I ebay the 135-400mm, the change over price may be worth it to gain the extra two stops worth when hand holding.

The 135-400mm isn't a bad lens optically. There are no noticable distortions or CA (unless you pixel peep and look for them). Compared to my 70-200mm F4L, the sigma will always come off second best.
But for what it is and comparing it to other consumer grade lens, its pretty good. Optically the only real downer is that it's a bit soft over around 340-350mm but stopping down to f9.0 fixes that. I've taken moon shots with this lens @ f11 and gotten good crater detail right across the moon face.

The problems with the 135-400mm are barrel creep, focus speed (compared to my other lens) and the lack of OS. The new 120-400mm looks like it solves all those problems so this lens looks like being a bit of a winner.

BTW, what price did Harvey Normans have the lens for?