In my experience I've found that the Buy It Now option works best in combination with an auction price on second-hand big ticket items, like dining tables, TVs etc. This is probably because some people need those items urgently, and are prepared to pay a bit more to secure it. I've also found they work quite well on second-hand books.
A Buy It Now can also be a good way to drive the auction bidding up, as it gives people an indication of what the item is worth - e.g. you list a TV with a starting price of $400, and a Buy It Now of $750.
On the other hand, I Buy It Now's haven't worked at ALL for me on items like new and used DVDs and clothing! I'm guessing this is because there are a lot of sellers offering these things, and they tend to be items people want but don't 'need' - so the urgency is not the same.