I use Auctioneer also. I swear by it. As the previous poster stated, It uses YOUR servers prices, but only if you preform a scan first. As opposed to using say Thotbott for price checks, which pulls the 'average' cross server prices and may not reflect your servers economy. It really lets you stay current with the fluctuations in prices from any given day/week to get the most buck for your junk. ^.^
I also have a "bank" character that I use to do Guild bank, as well as bank work for my toons. Its invaluable to me.
You only need to run the scan once a day, on any character as the prices will show up on any character with Auctioneer loaded, as long as they are the same faction/server.
I run a scan every morning, usually while I go get food or start the laundry, when I'm done so is the scan. Easy.
It comes in handy for Guild or Trade chat Price Checks or when deciding resale prices for anything, loot drops, profession made goods, stackable goods etc. It also helps you save money by never having to pay more than you should, since the prices are right there at your fingertips.
I personally use Auctioneer Classic, it was the first I used and I got used to the clean look of it, but its really up to your own taste.
Auctioneer gets 2 thumbs up from me.
Happy Gaming!