Olopatadine hydrochloride is an antihistamine (as well as anticholinergic) and mast cell stabilizer. 

Its 0.1% solution is sold in Australia as Patanol.

It is a relatively selective H1-receptor antagonist and inhibitor of histamine release from the mast cell. [Source]

Preservative: benzalkonium chloride (which is a problem).

What it does (academic studies)

FDA's clinical review (PDF) – this is very detailed.

Other studies

Both laboratory and clinical studies have established the efficacy, safety and comfort of olopatadine in several study design models and comparisons to other antiallergy medications. The application of olopatadine, specifically in the management of lid swelling, an allergic sign recalcitrant to therapy and nasal allergic symptoms has also been established. [Source]

The drug effectively relieves itching, redness, chemosis, tearing and lid swelling related to ocular allergy, and is also helpful in the management of nasal allergic symptoms. [Source]

My experience

My experience was not too bad, but there was only a very small improvement. Not worthwhile.