I am moving to Switzerland next week and need to find an apartment asap. We were thinking of living in Thalwil - have not been there but it has come highly recommended.
Thalwil is a lovely place to live, but unless you have a) loads of cash (>4000Chf per month at least to spend on rent) or b) fluent Swiss-German it is unwise to restrict your apartment search to such a small area.
You are better concentration on a larger area - eg the whole west side of the Lake from Wollishofen (in Zurich, on the tram routes), to Wadenswil (20 mins by train) or even Pfaffikon SZ (about 30mins).