For anyone thinking of buying a bluetooth intercom set here's my experience of the Lexin B4FM that I've been using for the past couple of months.
I bought a pair from Amazon UK based primarily on the large number of positive user reviews. The pair cost £150, so they lie somewhere between the cheap unknown brand sets and the more expensive Cardo and Senna sets. The Lexin's come with both boom and loose wire mics., so no extra cost here if you need either option.
Fitting one to my AGV-K3 was fairly straight forward using the supplied screw-together clamp that grips the helmet shell. As supplied, the clamp did not feel super secure, so I cut a piece of foam rubber to match the contour of the helmet and act as a spacer inside the clamp. This mod made the fitment rock solid.
Same approach was used for my wife's modular MTS helmet, although in this case the boom mic was used. One problem with the MTS is that the tinted visor controls sit at the bottom left side of the helmet, so I had to mount the intercom on the right side instead. I had some concerns that this might cause a problem with water getting into the electronics as the USB-C connector used for the mic and headset now faces forwards, but so far it has been used through some serious rain with no issues.
Battery life is good - running them for a couple of long-ride days doesn't even register a battery drop. Audio quality is good, with wind noise all but removed when talking to a listener on a phone call or on a connected intercom. Headset volume also goes very high - painfully high if not using earplugs, but this is needed if I'm using the silicone wax earplugs that block a lot of sound. I can't really report on range as I don't use the intercoms for communication in a group of riders, only for rider to pillion, phone calls and GPS directions.