I have used a few different rigs with the C300 many of which were good. The most important thing for me when buying the rig was getting something modular, I didn't want to invest in a rig for a camera which would be useless when I changed camera, I wanted something that I could add to and adapt to different shooting scenarios with different cameras. Most of the top name rigs work in this way, including Redrock, Zacuto and Vocas.
This picture on the left was the rig I started out with when the C300 first came out. This consists of Redrock parts with a manfrotto quick release plate to mount the camera directly to the Redrock shoulder rig. The advantage of this rig is having a comfortable shoulder pad - great for long shoots with lots of hand held work. This worked ok, but the camera always felt a bit high on the shoulder, the C300 is already a fairly top heavy camera so I decided to change things around to bring the camera lower down.
On the right is my current set up, it is much the same as before, although I here use a Vocas riser to mount the camera. The vocas riser allows the camera to slide onto the bars, bringing
the camera down closer to my eye line. There is no shoulder pad, I decided against it as it is perfectly stable without one, by pushing forward on the bars a little the weighted block at the back sit snugly onto your back and keeps the camera stable. I also removed one of the weighted blocks as it wasn't needed and creates a nice light rig without it.
The vocas riser allows you to slide in 15mm rails, which give you the correct height to add on a matte box and follow focus. It is a pretty solid steel construction and has movable screw holes on both top and bottom, so you can slide the camera or tipod wedge into the correct position. I tend to leave this on the camera all the time and slide the rails in whenever I am doing handheld work.
Another big benefit of the Vocas riser is the screw holes on both sides, these are great for attaching monitors etc. I often add my monitor using a Noga Arm, this is useful as you can set the LCD screen to view assist and feed the Clog picture into the monitor. It is also useful when you don't have a large dedicate client/director monitor, this way they can just look over your shoulder and see what you are shooting.
The Redrock rig packs down very small if needed, I break it down into just 4 parts, so it is quick to assemble. I have used Zacutto rigs that were broken down into so many parts it was like some kind of Maccano set, and took an age to build - not idea at the beginning of every shoot.
The Zacuto Stinger is another popular rig for the C300. The good point about this is that the shoulder pad is low on the bars, so the camera isn't mounted too high up. I am not a huge fan of this rig, simply becuause of the distance the hand grips are from the lens and the camera. They are fairly comfortable to hold like this, especially with the ball sockets in the handles which help with positioning, but when you want to quickly reach up to focus, the camera seems miles away. This could just be a complaint personal to me though.
Another rig that I have used that is worth looking at is from Tilta. Tilta are a Chinese company, but there gear is much better quality than a lot of the Chinese made video gear on the market. The rig has a small plate that fits onto the camera and then slides onto the Tilta plate, which in tern mounts onto a normal V mount quick release plate (those designed for standard Sony ENG style cameras). There are holes at the front and rear for adding bars for rear weights, v lock battery adapters and hand grips. The big plus about these tilta rigs is the price. The item to the left goes for 430 pounds in the UK, add some hand grips and rods, and you will still have something costing less than a similar sized rig from Zacuto or Redrock.
If I had any complaints about this rig it would be that the camera doesn't lock in quite as tightly to the mount as I would like. The camera is held in place by a screw fitting which can come a little lose causing a slight wobble on the camera. This however, is a fairly minor complaint as you just have to ensure you keep tightening the screw when it loosens.