I have been using wood casks for aging both straight and sour beers. With a new oak cask, I begin by getting the cask watertight. This involves filling, waiting, and refilling until it stops leaking. My last experience with this was a 15 gallon american oak barrel which took a week to hold water. After this initial treatment I follow the advice from the book "Lambic", and refill the barrel with water treated with approximately 1 Tablespoon of soda ash per gallon of water letting it soak and leeching a bit of the strong oak flavor out (soda ash/washing soda works a lot like baking soda in this regard). This procedure can be repeated weekly to scrub out the harsher oak flavors. After a few weeks treatment, I then switch over to a citric acid and sulfite (potassium metabisulfite) solution to sanitize the keg letting it soak at least a week before rinsing and adding beer.
My experience aging beers in wooden kegs is limited, but I am currently conditioning my fourth new barrel for aging beer so I may have more experience that some other homebrewers out there. I have noticed that the first batch of beer in a new keg picks up a strong wood flavor very quickly, and if a strong beer is going into the barrel it is even quicker. A strong IIPA I brewed and aged in a new 10g barrel had a very prominent oak character in a week, A 1.044 stout had a good oak character in about 2 or 3 weeks in a 10 gallon barrel. After aging the beer I cleaned the barrel using a commercial Oxy-PBW-type cleaner (basically soda ash plus peroxide) and then refilled with the citric/sulfite solution until ready for the next beer. The second and third batches in the same container picked up the same wood character over a much longer period. By the third batch of beer (even at 8%ABV) I begin to notice some sour notes, and start using the kegs for long term aging of sour beers (this is my entire reason for the enterprise).
From what I have read, the smaller kegs will transfer more of the wood character more quickly. My smallest barrel is 10 gallons, and this fits my batch sizes well. If I found a deal on a 5 gallon keg I will give it a try, but I basically found bargains on the ones I have, and can't afford new kegs at this juncture. For what its worth I have heard good things about products from the barrel mill and gibbs brothers cooperage. I am not sure of the source of my barrels, but they are likely from one of these two places. They had all sat for a number of years before I convinced their respective shop owners to get rid of them on a close out price. If you can find a deal on a barrel, even if it has been sitting dry for a number of years, you can likely soak it back into shape. I would recommend trying it out.