I got into KB's in the mid-90's thanks to being able to do some Sambo training in Russia. KB's are not going to be good for building mass like lifting will. You will however get some gains, but more in the lines of muscular density as opposed to size. Meaning, ever seen those guys who look bigger than they really are? It's really their muscular fiber cross sections are so thick, they look huge. And make no mistake, their strength is through the roof.
KB's are amazing for functional strength, anaerobic endurance, muscle density, and not to forget, injury recovery. Supplement KB workouts with squats and dead lifts, and you will be a machine. I'm primarily a martial arts guy, and I use KB's as a mainstay, supplemented with the lifts that I mentioned.
If you want to get crazy strong and flexible, check out Bulgarian Bags. OMG... These things are evil. But they're probably the best piece of strength equipment I have ever used and purchased. I'm a big guy on the small end of the big guy scale (6', 220'ish) in good shape. I thought that I could handle a 34lb Bulgarian Bag... I lasted less than two minutes doing the most basic swings with it. So much so, I lost my grip, it launched, and put a hole in my wall. Ego checked... Ordered the 24lb bag which still kicks my ass.
My home gym has kb's, Bulgarian Bags, several mace's, steel Indian Clubs, gymnastic rings, and parrallets. I went this route about 5 years ago. And at 46, I have never, ever been in better shape. I'm in better shape now, than when I was an active duty Army grunt with a high op-tempo. I train because I'm a lifelong martial artist (35 years this September 2nd). I teach Brazilian Jiu-jitsu and Muay Thai. Which means I fight for real, at least 3-4 times per week, against younger and stronger opponents. This style of training is the leg up that never fails.