Personally, I quite enjoyed T3 for what it was. Sure, it wasn't a patch on T1 or T2, but it was an entertaining mindless blockbuster; which is what I expect when I go to see an Arnie movie.
Most of what I've read in negative reviews of this film are remarks about how it missed opportunities to go into deeper dramatic territory and the like. Whilst I wouldn't be opposed to a Terminator film getting all deep and meaningful in parts, it is - ultimately - not what we go and watch such a film for.
So long as its got a reasonably structured story and heart-pounding action scenes, I think it will have acheived what it meant to. Not all films are meant to be deep in the plot department!