I bought a Sable 2015 CNC engraver machine several years back. The original spindle it was delivered with at the time was extremely noisy and bad. Later a new spindle was available for it, and I upgraded to it. Now I can sit and work next to it while it is running (lets call it expected level of noise). The new spindle uses the same type of locking nut as the on in this machine it seems (ER11) which is really good in holding the drill/mill bit. It is also much more stable than the old one.
My machine did not come with any end-stop switches, but the hardware supported this so I have glued in place switches to ensure that I do not run the machine to far. Did not check this machine, but end-stop switches is in my opinion a must, especially since one can use these to home the machine to a good position, then use soft limits for the rest of the run (given that the software supports soft limits.
As mentioned above, a floating head would be nice for PCB work. I want one... My table top is not perfectly flat. Probably 100-200 microns difference across the table top. I solved this by adding a sacrificial wood surface on top of the bed and milled this flat compared to the milling head. It helped, but still there are issues. When putting a PCB on top and clamping it down you may add a small tension and curve into it, making milling a bit hard. A vacuum table top would probably solve this nicely as it will pull down on the entire surface of the PCB.
For holding down PCBs I would probably recommend to first start with standard sized copper clad PCB boards. Add a sacrificial wooden top, mill a pocked fitting the copper clad exactly, and standardize on this setup. Milling the pocked will fix any table top leveling issues. PCB will be at correct 90degrees angle each time. It makes alignment for double sided PCBs easier. One could in addition modify this setup to add vacuum to hold the PCB down. I have not done this yet, but it is on the todo list.
Another "issue" with my machine is that it did not come with any means for clearing chips from the milling process. It is also on my todo list to add a small air blower next to the milling/drilling bit to clear away dust and similar. Lubrication/cooling can also be added in case of milling metals or other material requiring that.
On the electronics and software side there seems to be big differences. I use a machine with LinuxCNC and controls the machine directly through the stepper drivers by the parallel port. The machine have to do all the realtime motor control (hence the special linux distro). I have also played a bit with arduino control. I used GRBL, which I assume this machine is also using a variant of. Actually worked quite a bit on a GUI with openGL support for GRBL, works but not finished. That said, this is more or less just about streaming G-code to a machine, so the more important is probably how the G-code is generated. For this I tried a lot of different tools, but finally found that bamcam,
http://www.cambam.info/ , was the most stable one for my use (and free for the 40 first projects). Also not to expensive if i want to buy it. Takes some time to learn to use it, but very capable software when one learns to use it.
So, what are my experiences with milling PCBs?
I only consider it for prototyping use or through hole projects or projects with large surface mount devices (0805 and bigger). Trying to do super thin traces and isolation is a pain. If the table top is not perfectly flat it will be bad. If the PCB curves a bit while strapped down it will be bad. Due to this I prefer through hole projects or SMD pars where the isolation tracks can be fairly wide. Using a flat 0.8mm endmill is no problem. Bought a couple of 0.2mm endmills and broke them both in 10minutes. V-type milling bits works great in some cases, but you do need to consider the fact that they are V-shaped when calculating cutting depth and width of traces and isolation. Flat end-mills are much easier to use as it is easier to see how the final results will be in the CAM software.
I still consider my CNC a useful machine, but it takes time to learn and know its capabilities and how to setup what you are trying to make. Given how much I use the machine I'm not sure I ever will save money compared to just ordering the PCBs online instead, but since I like to make stuff, I consider this as part of my hobby and dont mind this fact

One nice thing is that drilling perfect holes in the PCB becomes very easy.
An enclosure for the machine is a must in case you do not want to have a mess to clean up (or dust to breathe in).