I'd steer clear of the Double Tap derringers. Heard too many reports of folks having wayyyyy too many problems, such as light strikes or double-firing both cylinders simultaneously, in addition to the fact that the recoil is just plain brutal with those things. (The ported models are only slightly less painful than the non-ported.) If you want a tiny, thin, lightweight BUG that will go bang every time, shoots ammo that penetrates adequately, holds 7+1 rounds, and won't beat the snot out of your hand, get a Kel-Tec P32 and load it with spicy FMJ's (Fiocchi or Sellier & Bellot). Otherwise, if you're looking for a "just because" derringer, your best bets are going to be those offered by Bond Arms (if you want big-bore, mule-kickin' fun) or North American Arms (if you want adorable little .22-caliber fun things).
The Bond Arms derringers are, as others have mentioned, huge and heavy, but they are VERY well-made and tank-like. Probably makes a very good impact weapon if the first two shots don't do the job.
The ability to buy a whole slew of different calibers (they pretty much all fit the same frames) and lengths of barrels, and swap them out with just a single screw, is also pretty nifty. They're kinda pricey, but worth the money in terms of build quality. But in terms of being a worthwhile CCW piece ... ehhhhhhhh ... not so much. Go to your favorite LGS, pick up a Bond Arms Ranger or Snake Slayer .45/.410 model, and then compare it to one of many single-stack pistols - M&P Shield, Glock 43, PPS, XDS, PF9, LC9S, etc. The Bond Arms derringers weigh more and not only have just two shots, but those shots are slow and awkward because it's a single-action piece and it's not quite as easy to operate as, say, a Ruger Blackhawk.
The whole "the bad guy won't count your shots" argument just doesn't fly with me; I'm not looking to INTIMIDATE a bad guy if I draw a gun, I'm looking to STOP THE THREAT ... ***NOW***. Hits aren't guaranteed, and bad guys don't always travel alone, so packing a derringer as a primary is inadvisable, at best. Even in the role of a BUG, again there are smaller, thinner, lighter options that hold more rounds and work equally well or better and have a less awkward manual of arms than a single-action derringer.
But in the role of a range toy or something for snakes ... welllllllll ... honestly, the Bond Arms derringers are tough to beat. I was kinda tempted to pick up a Heizer Defense Pocket Shotgun, which is a single-shot .45/.410 derringer (which had a small role in one or two of my books
), partly for something to have in my pocket for snakes for those rare times I'd go out desert trail hiking, but mostly just for giggles. But I realized that something that thin and light and firing a cartridge like .45 LC or .410 shotshells is going to SUCK in terms of felt recoil. But there again, it offers the option of also being able to swap on other barrels for .223 and 7.62x39 (the "Pocket AR" and "Pocket AK," respectively), which is kinda neat ... but also sounds even more unpleasant. Sooooooo ... just putting that out there, FWIW.