Just got back from 2 days of using my Sena SMH10 clipped on a Shoei Qwest fullface. Had no expectations as this is my 1st comm system.
Read reviews, watched videos on Revzilla.com and this seemed a good system for the money that doesn't drown the operator in too many options and little buttons.
The side mounted jog-dial/volume control is freakin amazing. Spin to adjust volume and functions, push in to activate. Riding in heavy 5-lane traffic you don't want to think about fumbling for a stupid button or switch.
Paired to my iPhone 4 as phone and iPod it worked wonderfully. I needed to add the included speaker pads behind the speakers to bring them closer to my ears, otherwise volume was pretty low.
As others have said, phone quality is excellent and no one believed I was on a bike and conversing. I'll be careful how I say this, but at 75 and (ahem) above
I could have a fine conversation and maintain all my focus on the road. With the iPhone's voice dial feature I can dial anyone in my contacts list at will.
Music quality was average, there's really no bass to speak of but it was fine with me and I enjoyed having tunes on the ride. There are better speakers I'm sure, but it's does the job.
(Note: singing along inside your helmet sounds awful, I'm gonna say it's the acoustics in there... not my voice)
Have yet to pair it with another rider but I expect that will go well since vocal communications are quite clear, just not music.
Don't have Bluetooth GPS so no experience with that.