Relative newbie to Sena 20s here. Firstly, I had no prior comms whatsoever, so no other comm system for comparison, and a lot of my good review stems from the first opportunity for music and conversation inside my hat.
1) Easy to use? Yes and no. Yes, easy to install; easy to charge; easy to listen to iPhone music and FM radio; easy to change stations/songs; and easy to sync with other Senas (haven't tried with other brands). No, because there are so many features that my tiny brain can't remember all the button-pressing sequences to do all the potential things. The 'cheat sheet' helps, but not at 60 mph on an Atlanta freeway.
2) Pairing? After the initial 'consummation' with the iPhone, turn on the phone first and the Sena mates automatically during startup. After the first 'hubba hubba' with other Senas, my Sena mated with them again during startup. Battery lasted for over 7 hours of more-or-less nonstop conversation with rpm on a Big Money Rally run through middle Georgia. Had a threeway with MrZappo and his buddy when they rolled through on their way to Daytona Bike Week, and all three of us hooked up within 30 seconds. (All Senas, which surely makes a difference.)
3) Loudness? Not enough. But that can be said for my Subaru Outback, too. It helped when I inserted the (included) shims to put the speakers slap up against my ears, but Rage Against the Machine was meant to be played with a few more dB than the Sena provides. Frank (Sinatra) sounds just fine, as does Miles, Diana Krall, and acoustic Neil Young.
4) Sound quality? Let's be real, here. It ain't a 400-watt Sansui with Advent speakers. It's a battery-powered comm system with super-thin speakers the size of silver dollars. That said, I can hear what's playing. Had some difficulty listening to Georgia Tech baseball play-by-play on the radio, but I was traveling at 70 mph ... which brings me to my last point ...
5) I have become acutely aware of the noise created by my helmet. (Bell Vortex; see avatar.) It's not wind through the neck opening. (Scarf test.) It's not wind through the vents. (Open/closed = same noise level.) It's just wind turbulence on the outside of the helmet that's transmitted directly through the shell. Wind turbulence (because it changes loudness and pitch as I slowly move my head around) transmitted through the shell -- because how else am I hearing it?
So, maybe a new hat will show up on my next birthday, and King Crimson will once again blast as loudly as they did on stage back in 1972.
Would I recommend Senas? Yes, without hesitation. Are they the best? Don't know, having no experience with its competitors. Something to be said for having a unit that is common among other riders, though.