I have the Garmin 660. It is water proof so that it can operate out in the elements on a motorcycle. The waterproof satnav's are usually much more expensive than ones designed for in car use. The 660 operates like the 550 but has a larger screen and as it has a flatter shape it is more convenient to put in your pocket when you stop.
The maps are good and the “Lane Assist” feature is good on motorway junctions. It shows the junction and clearly indicates the lane you require with a BIG arrow.
I sometimes use it without audio and I am able to navigate with it OK. You just need to remember to look at the display more often.
With audio you get plenty of notice before a turn so you can concentrate on your riding.
Wind noise is a problem with most/all helmets. I have poor hearing so I always wear ear plugs to protect what hearing I have left.
When I want audio I use Etymotic ER-6 Isolator In-Ear Earphones. More information at:
http://tinyurl.com/7tj2s
These are the type of thing that you would use with an mp3 player or Walkman and offer high audio quality. They have flanged ear tips that push into the ear cavity. These are the best ear plugs I have ever used for reduction of wind/road noise. Even the Rapide seems quiet! Really. They are not cheap. You can find them online for around £60 - £70. There are probably similar, cheaper alternatives if you do a bit of research. They are far, far better from a noise reduction point of view than the custom made moulded ear plugs that I had made at a bike show. (These weren't particularly cheap either.)
The Garmin kit came with a car mount too so it is a moments work to unclip from the bike and stick on the car windscreen. It operates on 12 volts but the inbuilt battery has enough capacity for around 4 hours if you only have 6 volts or are walking.
The unit also stores mp3 files so you can listen to the music of your choice as you serenely waft along in apparent silence.