Just about anything that captures video will work, of course. If you want to slow it down at all and analyze it, you would want something with a fast frame rate. A lot of cameras shoot 24 or 30 frames per second, but for slowing down high motion I'd look for something that does 60 frames per second of faster! Also a lot of cameras are now including high speed shooting modes that shoot at a lower resolution but higher frame rate (120 - 240). One last tip: find a dark background to film against and use the brightest fly line you can find!
Something you might want to think about is back ground while filming. many of the casting instruction videos the people use "white rods" and cast with a dark back ground behind them with a "bright fly line" getting higher then the caster also helps to take the sky out of the film, it's pretty darn hard to watch a fly line in the sky moving at the speed it does. I would suggest a small latter getting slightly above caster from behind on his off side casting shoulder for safety reasons. you can zoom in for forward loops and zoom out for back loops and the whole motion. trying to get the whole motion from the side and the whole line movement from all the way to the straight line on back casts and all the way to the straight line on the forward cast can put you to far from the caster making it look like it's way to far away. hope that helps!