... How bad is it really, in Beijing and Shanghai in particular? I've heard I should bring Claritin, Sudafed, Visine (I'm a contact lens wearer) - will any of these things help? Tips most appreciated - thanks.

None of those.

If you are staying under 2 weeks, you shouldn't have any problem.

2 weeks or more, you'll want cough drops.

The smog doesn't cause allergies, nor does it create problem for your eyes.

that's not true. smog does affect eyes if they are sensitive. i don't know if visine helps, but eyes may hurt or water/tear up. Pollution in Beijing can be pretty bad - don't know about seasonality. Some people are allergic to fine dust particle may be in the air. i guess it wouldn't hurt to bring those things as they are pretty light and if not needed it's not a big deal. but if needed, you wish you had brought them.

we had no trouble at all in beijing

Beijing does get sandstorm, and you may have problem with that. Even locals will wear masks on days with it. But it's not the smog that's around almost anytime of the year.