The problem with this, at least in NYC, is that apartments go so quickly. There's no way a good apartment will be available long enough for this to get back to you.

You could leave an e-mail address, as well.

When we were looking for a house, we asked the people who would be living above us. We ended up living somewhere else (due to issues that we were previously aware of), but they were very friendly and informative.

But your idea is indeed less invasive.

That's a great idea and creates a win-win situation!

Sadly, NYC apartment hunting is a little different. Landlords and apartments have a huge incentive to use their limited advertising bandwidth on already vacant apartments.

Vacant apartments cause landlords to bleed more money than almost-vacant apartments, and brokers are all about selling quickly in volume, which is easier to do when the aparmtent is vacant (easy to photograph, easy to show)

That's very interesting. So really the trick is to somehow catch people to write a review before they leave. Of course the problem is to post it publicly would expose their identity (since they have a unique address) and possibly open them up to retribution from the landlord. So a kind of 'Yelp for Apartments' really doesn't seem like it would work.

One possible solution is to take advantage of the likelihood the last tenant still has forwarding setup with the USPS. If you could snoop out their last name, you could send the note addressed to them at the apartment and it would likely be forwarded to them where ever they went.