|An Emerson folding karambit|
Unlike many martial arts weapons, the karambit remains an excellent choice for a backup weapon in modern combat. In particular, the karambit is useful as a close-quarters backup to a handgun. Someone grabs for your handgun--hold onto it with one hand, draw the karambit with the other hand and begin cutting important stuff. Your opponent is unlikely to hang onto your handgun for long.
Honeybadger (pricey, custom) is purpose built for this kind of use.
Generally, a fixed blade is preferable to a folding knife, but many states have laws against concealed carry of fixed blades, and also limit the length of a folding knife carried. Under these circumstances, a waved folding karambit like the Emerson is an excellent choice. The wave (the hook on the back edge of the blade) flips the blade open during the draw, giving you the next-fastest thing to a fixed blade. And, karambits do not rely on blade length for their effectiveness - the Emerson karambit, as an example, has a 2.6 inch blade length, which is well under the legal limits imposed by most locales.
Karambits do definitely lose out a bit on utility purposes, though they will open packages and envelopes exceedingly well. I carry mine solely for defensive purposes and have less-scary knives for every day utility tasks.
The karambit is very intuitive to use, but, like most things, the effectiveness sky rockets with a bit of training. Pretty much all schools of FMA have karambit techniques that they teach. Here's a YouTube clip that provides a good sampling, performed by Guro Doug Marcaida. I like the student's expression at about 1:31 - he knows he's done for.
Most knife makers have made a folding karambit at one point or another, though the Emerson karambit is the standard that all others are measured to. The reverse grip of the karambit means that a wave feature is pretty much a must-have for rapid deployment. If the Emerson is out of your price range (at over $200, they ain't cheap), I'd check out the Fox karambit, which are around the $130 mark. I'm not aware of any other quality made karambits that also have the wave feature.
For fixed karambits, the options open up quite a bit more. A proper sheath will be critical. If you have the coin, there are some very nice models from custom makers.
Because of its unique characteristics, many armed professionals are beginning to train with and carry the karambit - from police officers to SEALs and others. For armed citizens, it can make an excellent back up/retention tool to a concealed handgun. Definitely worth a look.