I actually enjoy T3. It's a fun movie with a terrific ending, I think.
The main problem I have with it is two things really. For one, I don't buy Stahl as Connor for one minute. He just doesn't work for me in the part at all. I think it's such a shame they couldn't get Furlong to do this since I think he really worked very, very well in T2. Stahl's Connor comes across as nothing more than a loser, really, IMHO. And while Connor is certainly a messed-up character, he's also a hero to some extent. And that just doesn't show here at all, I think.
My second complaint is atmosphere. T1 and T2 both have an atmosphere of fear, desperation and breathlessness. T2's opening scenes with the highway soaked in orange and with that foreboding music is a prime example, I think. But they've also got a dose of hope (T2 more so than T1) as well as some humour in T2.
In T3, except for the ending, I just couldn't find that special atmosphere. It's got enough action, some of the humour is good...but without the atmosphere it just doesn't quite live up to its predecessors, I think.
Still, again, I do enjoy it for what it is. And I certainly think it's entertaining.