Im a little biased but i would say they are worth it, even though they can be expensive. I love using mine and have been really getting into them more and more over the last two years.
There is a huge variety of exercises you can do with one kettlebell, even more if you have two of the same weight. I incorporate kettlebells with other equipment on a regular basis as well. They can also make a great "travel gym" if you are on the road and don't want to miss a workout. I usually take one or two any time I drive out of town for several days. Your personal value may be different depending on what equipment you already have and your level of strength and fitness. You can make gains in both areas but for pure strength training, it may be best to invest in some good free weights and a bar first. By comparison, KB's are not that expensive. Another plus, as martial artists, kettelbells work in a lot of familiar movements such as rotation and hip drive, pushing and pulling similar to what it feels like when using striking and throwing techniques and so on.
If you do decide on KB's, one way to ensure getting the most for your money is to get a good size. A heavy bell will be more expensive but if you find the right size it will save you from buying a light one, then buying another one shortly after you start using them. Find a gym or a friend who will let you try out a few sizes. Try a few workouts and estimate from there. Everyone has different strength, and endurance limits.
I have started kettlebells with a coach that lives near by, and .............. WOW, really nice workout mainly doing swings and the turkish get up at the moment, right now i am loving the TGU, about as technical as a martial artss form, good stuff