I have a fair amount of confidence in my product knowledge for the type of properties I represent and the buyers that I typically work with.  I would categorize my expertise as sub-million condominium properties.

In Manhattan our market is broken down into three categories, condominium, co-op, and townhouses.  Townhouses represent a very small subset but specialized part of the market, co-ops represent about 75% of the market and the rest are condominiums.

Within my areas of expertise I know what properties sell for, I know the listings that are the most attractive, I know whats just came on the market.  I take time to go see these properties so I know which ones have the good views, which ones need the gut renovation, and the ones the agents say the seller is ready to look at all offers. 

What I have found is that during open houses or appointments I can have much more intelligent conversation with attendees or discussions on the market conditions. 

It's fun to talk to people and ask them what they think of other apartments having seen them myself. Once I can talk to somebody and if they share what they are looking for its fairly easy to know what will work for them at the given time.  The people who want open views, dog friendly buildings, washer dryers in the unit, whatever the preferences might be, its easy to have a conversation, rather then just sitting there in the open house and hope that they are going to fall in love with the apartment on the spot.

It seems to me that previewing is a lost art, only for the "new" agents who have nothing else to do.  Naturally every body gets really busy from time to time and its not easy to set time apart to go look, especially in Manhattan where we don't have lock boxes.  Taking a Sunday and going on the same open house circuit that other buyers are going on is just as worthwhile probably more so than holding a random open house somewhere and hoping that people show up.