The only one I have experience with is the Harbor Freight mini mill (not to be confused with their micro mill). It worked well for my intended tasks, and was fairly accurate. I really like the adjustable speed with the turn of a dial.
But it all depends on what your goal with it is. These small mills are not made to remove much material so if you plan on doing tasks like slotting guards, hogging out integrals, etc they won't really help all that much. I found it easier to just use files for slotting guards instead of the mini mill. For those tasks a machine with more mass is needed to do the job efficiently.
I used mine mainly for more accurate drilling compared to my drill press, milling out material like wood, G10, etc., and light metal removal....for those tasks I was happy with the HF model.
