I have tried a few ESCs with FOC or similar system on it and I really like it. You can keep like 10-15% throttle down on the transmitter and basically it just slow crawls up stuff with no change in throttle. I can watch it on my transmitter, just put enough throttle down to get it moving and it will keep that speed unless you adjust it. If you do adjust it, the adjustments are smooth if it is designed right.
I think where people get thrown off by it is the transition from FOC to normal running mode, it can lurch if you aren't careful. If you are also in the habit of adjusting throttle to crawl over stuff slowly and avoid stalling, it is going to take adjustment, but TBH I felt it was really easy to get used to. You don't get any feedback if it is going to stall in that lower throttle range, but it never does, so it doesn't really matter. I think people looking for feedback from the motor and that want to manually adjust throttle wouldn't get along with it.
I have a Fusion Pro and it works really well, the transition off of it is smooth enough that it won't lurch or jump forward unless you make a big mistake. I can gradually add throttle and it is barely obvious when it moves off FOC mode. I'm considering pulling the Castles I have in my trail trucks out and going to this, if anything just for simplicity, but the performance is really impressive IMO. The form factor is nice also, if you take a trip and have an issue with your motor or ESC, you can just yank it out and not worry about carrying a bunch of other stuff, this saved me last weekend when I drove over an hour to crawl and my ESC blew up 10min in, I had this one, tossed it in, then went driving.
The Crawlmaster Mini has something similar and is slightly smoother I feel, but they are difficult to find, you have to be able to solder it yourself, and they aren't weather/waterproof. I think the throttle curve is more sensible on these and it is one of the smoothest ESCs I've used, if you need to hop up on stuff then it's a smooth jump and it can handle 4s. My v1 blew up but I have a v2 here I've played with a bit and it is really smooth. I wouldn't use this in a trail truck though, probably just something for competitions or indoor crawling, it isn't waterproof and the exposed board could be a problem.
Furitek also has a similar system in the Lizard, but the transition from FOC is extremely jumpy. I also found it stalls a lot more. I wrote a little about this in the SCX10 Pro build thread, but overall I wasn't impressed. I tried it with 4 different motors and the only one it was OK on was the Holmes Revolver 540, it stunk on the others and was barely decent with that motor. Some folks have better luck, I think you may need to run their motors to get the best out of it? I'm not sure. I wouldn't recommend it either way, not based on my experiences anyway.
I THINK Silent Assassin has something similar, but could be wrong. He's on hiatus and I haven't been able to get one to try it out.
My only beef with the Fusion Pro is the lack of motor options and it is limited to 3s. The torque and low end on 3s is fine, but if you punch it, it lacks that oomph to get over things that 4s gives you. I think the FOC works really well in it and the amount of control it gives you is really nice.
People will rip me for this but I think Castles have become kindof antiquated compared to newer ESCs, they are great for when they came out and still good for some things, but they need to catch up with newer technology. If they came out with a silent ESC that had FOC, I'd be all over it, but there are better options out right now IMO.
That will make some good vid content! Perfect terrain in the Canyon to show the performance and capabilities of the combos.