Well, always good to see a new Darne thread! Along with that other classic French idiosyncrasy the Manufrance Ideal already mentioned, they are a bit of acquired taste. Some love them--in my experience generally people of good character, taste, and discrimination--while others don't. Vive la Difference!

For me, the ejection/extraction systems is one of the Darne's strong points. You pull the cocking lever up, the breech block slides back, and any newly emptied shell case is pulled back onto the action flats, ready to roll effortlessly onto the ground or into your waiting hand. Unless you're shooting driven game and your invitation for next season hinges on how many birds you harvest for the landowner's sale to the game vendor, auto ejectors have limited utilitarian value. Lots of guys find themselves stooping to pick the ejected empties out of the mud and taking more time than they would just lifting them out of conventional extractors and certainly more hassle than the Darne extraction system.

The R model Darne is a rugged, reliable time-tested design, usually found on light, agile upland doubles with superb balance and handling characteristics. I love my 20b R15 and generally keep track of what's on the market just in case something really cool shows up. A week from now I'll be in the Adirondacks grouse woods rotating between my Darne 20 (5# 9 oz) and my MF Ideal 16 (6# 1 oz). I know it's better to stick with just one gun, but I have trouble deciding between these classics, even though the Ideal is the better specimen of its kind.

Regarding stock dimensions, quite a few Darnes have pretty generic specs-- DAC around 1.5", DAH around 2.5", little if any cast, and LOP somewhere between 14" and 15". Like Ted says, it pays to ask before taking the plunge, but shooters who do well with "average" stock dimensions have a decent chance of finding a Darne that fits.

Besides stock dimensions, the major pitfalls in buying a Darne are the same as with other double guns--pitted or over-lapped out barrels, cracks in the wood that may or may not be apparent, mechanical soundness, etc. Finding a good double gun expert who knows his (or her) Darnes to check it out before or soon after buying is a good idea. I ignored that advice when I got my 20, and 3 years later discovered that a critical part was broken when the thing just stopped working.