Recently Trading Places' own Gary Butler was interviewed by the Guild of Professional Estate Agents on the subject of "open house" viewings to sell your property.
An open house viewing day/event strategy is only beneficial in certain circumstances. Firstly, the market, or at least the property in question, should warrant an open day e.g. there should be sufficient buyer demand.
We have found that by allocating specific viewing time slots, you allow prospective buyers their own time to take everything in.