These are of course expensive tools, and in my experience you truly get what you pay for. I've had the opportunity to use Preston, C-Motion, Arri LCS, Bartech, Viewfactor and a bunch of older units like the Genio and a few others I forget their names.
In the low cost area I guess we are currently speaking of the Viewfactor System and the Bartech. I have had difficulty with the Viewfactor system in wireless mode... not to dis or throw bombs, but recent experience left me pulling my hair out in frustration to the point where I just hooked it up hardwired and left it at that.
As for the Bartech BFD.... I DP'd a show in 07, shooting S16 and got my F-Puller a Bartech as the lower cost option to a Preston. I had done 3 shows with that fine gentleman, and I knew he was a damn good focus puller. Why then did we get back dailies that were consistently soft focus? Because - folks would be getting cell phone calls on set, and the RF interference would freak out the system and send it off into terra incognita. Ooof. Found this out 2 weeks in to a 3 week show, and I had been defending my guy to production that whole time. He nearly lost his job over it, and that is a big deal. Said to myself, "Never Again" and that was that
C-Motion's are awesome - very smartly put together, and ergonomically friendly. No issues with wireless, motors, cables or anything. The Integrated LCD that gives you a virtual lens along with depth of field tables was the best I had ever used. Used them 3 times as F-Puller on features - last one was a steadicam job that had me constantly running, grabbing sled from the op, running for marks and was very physically demanding. You tend to wear the handset on a lanyard around your neck, and this I did. The LCD add-on did not like this at all, and the electronic connections at the junction point became intermittent and ultimately failed. Perhaps they have beefed up the hardware and the connection strength since then. But it was damn nice up until then... could set electronic marks for the lens, do iris pulls with ease. Verrrry sexy.
Prestons. What can you say. They are bomber. Bulletproof. The industry standard, and worth every penny. Several career 1st AC's I have worked with have purchased their own systems and use them on every job, renting the units to productions. I still haven't learned all the secrets the current handset can offer, and when I have one on a job I breathe a sigh of relief.
Currently working as A camera 1st on a large budget film, with a famous director and A list talent. Can't go into too much detail because of NDD issues. But we are using the Arri LCS system - which may be my new favorite. Very simple, rugged and reliable.
The handset is small! The batteries last all day. 2 camera show, and I had to get at the frequency control to make sure A & B cameras were on separate channels - very easy. Got to admit, my favorite feature is this awesome little bracket that hangs off the rods right were your normal FF would sit, and you can dock the unit in there.
3 weeks in and I haven't pulled the FF out of the box once, just prefer to stay on the LCS - Very handy to do so to, as the director likes to be at camera working off the onboard monitor, so I just respectfully fade back and work the shot without getting in his way.
Just my experiences with these things... When it comes to focus - brothers and sisters, don't skimp... don't worry about how much it costs. Get the tools that will help you keep your job. As for purchase - the same applies - Gotta say I have been very envious to learn that some AC's pull in that extra $300/day for a 3 Axis system like a Preston, Arri or C-Motion as a tidy kit rental. My opinion is that the Preston's stand the test of time and would be the best choice for long term ownership and ROI.