I have the full set of Keltons and have really enjoyed them.
For the person who was having difficulties with them chattering...
So long as I observe a few things while working with them they seem to be really stable tools.
First, the actual cutting edge needs to be onthe midline of the bowl, when the whole tool is horizontal, include the curved section if you are using the curved tools. I finally started using a set number of washers on my tool rest post so I could quickly reproduce the right height.
Once that height is set, the agressiveness of the cut is adjusted by where you rotate the cutting edge to, ranging from the center line and going down from there. Don't get above the center line or ugly things can happen.
Other things that affect chatter are
Reaching too far over the tool rest for the diameter of the tool, and the agressiveness of the cut. Either ease off on the cut, or use a thicker tool, or use a tool rest that lets you support the tool from inside the bowl so you are not hanging out so far. Another trick is to extend the handle out to counter then length of the overhang into the bowl.
Keeping the tool sharp is also key to no chatter. Remember it is essentially a scraper. It is the burr on the cutting edge that is doing the work.
Regarding having to have such a large opening compared to the diameter of the tool shaft? You can actually get away with a hole just bigger than the tool if you drill your hole first, then use the straight tool to angle in until you have enough hollow space in the bowl to work in. Then turn off the lathe, put a curved tool in by snaking it in, position it near where you want to cut and then turn the lathe back on. Remember to turn the lathe off when you are ready to remove the tool. That seems to work for me.
Lastly, the tool handles can be used with other manufacturers tools, and vice versa. Just pay attention to the diameter of shaft a given handle can work with.
Hope that helps!
Dave