I can't say I shop the AH enough to spot a great deal when I see one (I mostly just use the AH to offload all the BoE junk I collect, and Auctioneer watches prices for me easily enough), but I can appreciate the feeling frostaholic talks about here: just like opening up a loot window and seeing that drop you've always wanted, there's definitely a thrill in finding a long-awaited AH bargain.
And I don't know that we've talked about Bottomscanner in any depth before, but that's exactly what the addon is designed for-- given the price information from Auctioneer, you can hit a button in Bottomscanner, and the addon will scour the Auction House for any items that are priced for less than they're worth. It's automatic speculation, really, and while it only works because not everyone is using Auctioneer (because if everything was priced right, no one would find it cheap), it can be used to find good deals.
But still, there's nothing like finding a diamond in the AH rough yourself. Checking the AH for an item and then finding it one day for half the price you expected to pay-- that's an entire realm of excitement in this game by itself.