I run a generic system I put together with a Yagi (directional antenna) and a over stocked booster. I wanted to try before investing into the newer versions.
I agree with offgrid, it will only 'boost' what is there. For me I never saw 3g go to 4g, just a stronger 3g. May be my generic unit..
I assume most of the new antennas are already setup for the wide band of frequencies cell providers work on. My Yagi, I had to be careful I bought that covered the range (also wanted WIFI).
I found my Yagi to be very finicky for aiming, though typically stronger receivers, that issue along would make 'my' decision for omni directional. Again, maybe the newer ones are much better. LOL, I lost my Wifi when a big ol' Class A parked between me and the transmitter.
Do they work, yes. How well, just like the normal signal, depends where you are and what you can grab.
Just my experience with mine.