I love radar cruise control. Max distance on the Mazda is 4 or 5 seconds, my mom's ford fusion was the same. I haven't tried her Subaru's radar CC yet. I use it everywhere though, highway, local roads, etc.
The quality of the radar CC varies a bit between manufactures. Mazda's radar CC will accelerate normally but my biggest gripe is that it will allow your car to get a bit too close before coasting/slowing. I noticed if the closing speed is closer than 3mph, the Mazda will have to brake. It will also come to a complete stop and keep the brakes on for you if you let it. That took me a full year before I tried that though.
The fusion's radar CC was much smoother in acceleration and coasting when coming up to traffic but it also liked to hover 2-3MPH less than my set speed and then take another minute to get up to said speed (so if I had it set to 70mph, it would accelerate normally to 68mph, then take another minute to get up to 70mph.)
Both systems will turn off if the sensors are blocked, dirty, or in extreme weather. My Mazda's radar CC will turn off for under 19mph if there's a very strong glare when driving into the sun and will completely turn off if the sensor in the logo on the grill is blocked by say, snow.
FWIW: I've never had regular CC engine brake for me, only coasting.