I've just come back from a 3 day euro mini tour to Colditz 700 miles day 1, 200 miles day 2, 600 miles day 3, which is the first long test with the Sena unit, every other long ride ive used the starcom.
Here are my findings.
Installation of the mount is simple, its a bracket that clamps onto the side of your helmet, wires tuck away easily and look robust. I've fitted mine with no cuts to the skirt of my Shoei Multitec something i couldnt do with the Starcom. The original fitting mount with boom mic isnt long enough for my Shark Evo, but i purchased an additional mount with a longer boom for that and works great.
Paired my Zumo 550 with SD card music with my Phone and then paired with the sena.
Started the unit up at 0600 (UK)for the ride to Dover unit started beeping at 2000hrs (Germany) telling me battery was going flat. Unit was powered off for the ferry crossing. Running a USB lead from my powered tank bag to the unit, doesnt look great but it works (unused recharge time is 3 hours, add an extra hour or so if being used)
Sound quality was OK, but the 550 is mono where as the 660 is AD2P so music clarity will be better. Music direct bluetooth from my Blackberry 9900 is crystal clear.
Call quality is spotless, far superior to the starcom unit. (Ive also tested calls at 110 on a CBR600 and was clear to both me and the caller, to a point the caller didnt believe i was on a bike)
Bluetooth connectivity is simple and painless, and music still played to my helmet from my bike at 100m away.
If you get the B version (Amazon link i previously posted) Firmware updates to iron out little niggles are very simple.
I would always forget to replug myself into the starcom if i was on a scatter rally where im on and off the bike alot, that issue is no longer there with the Sena.
Volume control is easy, fast forwarding/pausing/rewinding of tracks is simple,using the phone function is also easy, Simply forward or back on the jog dial on the side of my helmet, no more reaching for the ugly volume control of the starcom, which i cant use if i have muffs on.
I have now ripped out my starcom where it will gather dust in the garage, love the Sena.
One added bonus is if you are only popping down the shop or doing a short run, you can pair your phone (with music on) to the Sena and still listen to tunes without messing around pluging in or using the horendous bluetooth unit Starcom use.
The other added bonus is they do umpteen different helmet fittings so if you have multiple helmets you can just simply add a new mount.
I haven't lost any quality on music or calls, i would say i have gained quality in both areas.
Hope thats helped, ive been as honest as i can, but it really is a brilliant system. I've probably rattled on long enough now so will stop boring you with my love of the Sena unit.