We've bought two, and both were good deals that came with some headaches.
NEVER pay for foreclosure listings. If you go to Hud.gov, you can get to a list of FHA foreclosures for your state. Also, use the MLS through a free site like Listingbook.com and look in the ownership field for those that are bank owned. And if you're working with a relator, they'll have access to foreclosure resources as well.
Keep in mind that with a foreclosure, you're buying the house as-is with no information about what repairs have been done or might be needed. Also keep in mind that the clean-out and winterization companies that many banks/agencies are using are not the best and brightest, and that can cause problems (As an example, our boiler needs replaced because it wasn't drained properly when the house was winterized). Make sure you have an in depth inspection done by an inspector with a good reputation before making any offers.
The offer process is a little different with foreclosures too, because you're not dealing directly with the home's owner. Some banks set an absolute minimum that the listing agent is aware of. Others look at every offer, which can take some time. In our experience, there's no back-and-forth of offer to counteroffer. Either your offer is accepted or rejected; you're free to make another offer, of course, but they don't give you anything to work with as far as what kind of offer they might be willing to accept. And of course there are no contingencies, so a good inspection is absolutely vital.
Closing issues can crop up too. The agency we bought the house we live in now from only uses one specific title company for the entire state, and they are super backed up. Our offer was accepted in March and we closed in July.
Auctions are a whole different creature than listed foreclosures, and have their own drawbacks. In most cases, the banks that own the auctioned properties have to accept the high bid, and that can take several weeks. So you attend the auction and if you're the high bidder, you leave a deposit in the form of a cashiers check, which is then tied up for sometimes as long as 3-4 weeks while waiting for the bank to make a decision. We attended one auction and decided it wasn't for us, after having $5000 of our money tied up for a month only to have the bank reject our bid.
Good luck! The foreclosure market can be a great place to get more house than you could otherwise afford, as long as you go into the process with your eyes open and don't mind the hassles and delays involved.