OK, this may be a bit long...
I think the best way to start looking at these products is to list what you have and what you want done. List your devices and requirements and then figure out what you need to have in terms of features and options to get things to work the way you need them to.
So, for me, I have: Arai Signet Q helmet, Garmin StreetPilot 2820 with XM antenna, supplying XM Radio / XM Weather / XM Traffic, personal cell phone (HTC One), cell phone acting as 'iPod' (Kyocera Hyrdo waterproof)
My preferred setup is to use the GPS, and therefore the HTC One syncs through the GPS which then acts a control hub for GPS and all cell phone operations. The Kyocera is to only supply MP3, as you can't stream music from the HTC cell phone through the GPS Bluetooth connection. ALSO: The 2820 has XM Radio but it does NOT send radio sound via Bluetooth; XM radio output is via cable connection ONLY.
So, what I'm looking for:
- minimum 10 hour battery life; 12 to 15 hours would be even better
- with shorter battery life comes requirement of charging while in operation
- 3.5mm cable input for XM radio
- LOUD helmet speakers
- ability to actually work with my devices
- MUST BE ABLE TO BE CHARGED AWAY FROM THE HELMET, my bike and an AC plug are not anywhere near one another.
I've got a much more complicated setup, and therefore a lot higher set of requirements, than most people I've been told. The manufacturer's reps were kind of negative to my needs, as if they were so far out, but I think that getting something that works isn't completely unreasonable. I looked at Sena, UClear and a few Cardos; we'll start with the Sena products. ACROSS ALL SENA PRODUCTS I felt the following:
- seems well made
- super features
- excellent sound quality
- high tech
- expensive
- ...not super loud

And that last one was, well, why I didn't end up buying one. Wasn't loud enough for me. But YMMV.
Sena 20S:

The most technologically advanced unit I looked at. Lots of features, 13 hours battery life, handlebar remote control capable.
OK, my thoughts:
Beautiful unit. Large as a house, but at least the battery life is great. The things that got me the most were cost and helmet clamp: it was the most expensive unit I looked at and the helmet clamp both extends below the rim of the helmet (even the clamp is large, but solid) and holds all the electrical pins for the interface to the helmet (see the gold contacts in the photo). So constant setting and removing the unit in the clamp, every time I charge, might wear out the clamp, as per
http://www.webbikeworld.com/motorcycle-intercoms/sena-20s/review.htmIMPORTANT EDIT: Read up on the 20S
before deciding to invest in it. Owner reports are very mixed - some people love it, some other people have problems ranging from complaints about too low output volume in music yet deafening volumes for system feedback beeps, to failed connectors.
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Since I have an Arai, I can go with the Sena 10U, an internally-mounted intercom that becomes completely hidden inside the shell.

Problem? You can't remove the batter to charge, it must stay in the helmet at all times. Probably not a problem for most (wasn't for me until last year), but that's a deal-breaker now. 10 hour battery life. Remote capable.
Super sano install, sounds like a standard Sena 10S which it is, in a way. Also available for a few other helmet models.
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Sena 10S:

10 hour battery life, otherwise a good unit with very good features and good (but not great) volume output. Remote capable.
Pretty good unit but not "exciting" in today's market, solid unit that is still holding its own.
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I looked at some Cardos but several different sales people, from completely different companies, weren't fond of them. So I looked but they seemed out of contention. Good looking but a bit plasticky.
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I surprised myself and bought one...a UClear 200

Why?
Size: all the photos I took here I scaled to the same size. The Uclear, if you can make out, is the smallest yet still retains 10 hour battery life. Can't charge while in use, though.
Sound quality: LOUD. And GOOD!!

Holy $@!&, good, and loud. Louder than the Senas, put them to shame.
Remote

Unlike the Sena remote this doesn't get in the way of the clutch lever, it stays on the grip via a Velcro wrap that goes over the grip then the remote sticks on top of that. Problem: I have small hands and the remote can get in the way of hitting out low-mounted turn signal. Rotating the remote into a certain position removes that problem for me; I'm going to try to move the remote to sit between the control pod and the mirror mount, so as the leave room for the heated grip & controller I'll be getting.
Problems? Getting the system to run precisely with my setup has been a learning experience. Had to update the firmware, which did not go well (don't ask!) but once I debugged the problems it updated and the remote started working.
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If you are OK with the volume, don't have a stupid-complex setup like I do, and are OK with the design and price...go with the Senas, I think. I still may try switching to a Sena if the UClear gives me problems, but I can't use the Sena remote with the heated grips I'll be using so boo

The UClear has AWESOME sound quality and super-easy install (no mikes for most people, they are built into the speakers!) and really small. Very well priced system.
The Cardos seems OK, didn't listen to them. Their cases felt a bit plasticky.
webBikeWorld has just covered the initial announcement of a brand-new series from Interphone, and the reps I was speaking to all spoke highly of Sena and Interphone as the best out there, so I may return the UClear and wait to decide what to do later.
I can say more later if you want.