I bought a Chinese made and got what I paid for. Slop in the threads is correct but the one I have, and I assume most others are like it, works on dove tail guides which have a free play adjustment which helps. If possible also pick one that allows you to lock it in position once you have it where you want it. They are made for machining metal and like all other tools for machining metal you can go from quality levels where accuracy is wishful thinking to accuracy that will satisfy NASA. Prices vary accordingly.
Someone I consider a master woodworker once told me that a master woodworker is not someone who never makes mistakes. He is someone who is able to cover them up so that no one can tell.