So think about that...
you bought a unit so you can have phone/music/maps in your ears. bonus is that you can talk to other riders. want to talk to someone on a different system? No problem, give up phone calls/music/maps and you can talk to them. What is the range of BT to a phone? Well, typically a phone is a BT class 2 device. You get 10M or 33'. How many bikes do you know that maintain a 33' distance when riding? Yeah, I'd go on a limb and call that "just doesn't work", but hey, if you had someone on the back seat, then it will work just fine between the 2 different units.
Sena communications with more than 2 units are subject to the following:
popcorn/sizzling/popping sounds at random intervals in most peoples headsets (again, never with just 2 people, but increase to 3 or more and this happens)
poor distance in BC's/Washington's/Orgeon's/California's terrain. Cardo is better, but still nowhere near as advertised.
drop out of distance and have trouble reconnecting to the group. This is even more apparent when you go beyond 4 riders in the group. the 20s are supposed to handle up to 8 riders. Good luck with that.
Other Sena issues:
not waterproof. I've had "rain fade" on the 20s and on the 10 as well. Luckily, I've not fried a unit in the rain, but have friends who did. No warranty service from Sena either.
Look, bottom line is that you need to keep in a group with the same units. When you finally get tired of shit not working on a Sena, then switch to Cardo and wonder why you didn't do it earlier. If you are just a single rider and will likely not connect to others, then any unit will work just fine. I found my Sena 20s to be fantastic with just me and music, or when paired to another 20s user (just 2 of us), the communications were great. Very HD quality and little to no noise. Again, go beyond just 2 people and the communications were shite. Pair a 10 and a 20 together...still shit, even with 2, but bearable. With the Cardo units, in the beginning we would ask "Is anyone still there?" because there was just complete silence, no mic noise, nothing. Then someone would say "yup still here" and we'd all go off on a tangent about how quiet these units are, etc.