I do if I found the hub to be useful and of value somehow.
One's that are kind of spammy or poorly worded/written I don't. The reasons being A) If the writer didn't take the time to make the hub solid I am not going to take the time to contribute to it either - I know this sounds harsh, but come on. Quality only helps all hubbers. I'm not talking about one or two little errors, I'm talking major spelling and grammar and akward wording - like a rough draft instead of a polished piece.
If I am in strong disagreement I tend to also bite my tongue as I recognize each persons right to believe what they will. The only exception to this is if someone asks "What do you think" etc.
If a hub is written well and I can see the writer was truly passionate and cared about the subject I always give it some sort of kudos and comment on it. I appreciate it when others do the same for me. It inspires me to keep writing and keep improving.