Here are my 2 cents:
I've been playing with using my android phone for a GPS, and here are my findings:
1) Vibration is fixed by having a decent quality mount (I used RAM). But my tests werent very long - few hours of riding each day for a week.
2) Data coverage is required for using Google Maps, so you'll need get a GPS app that downloads maps onto your device. I tested Co-Pilot and Sygic - both have their pluses, both have their shortcomings, but neither matches functionality of a decent Garmin unit (see below). Believe it or not, one of these "no data connection required" apps (i don't remember which one, but I want to say CoPilot) refused to start because i didnt have data connection and couldnt check the license.
3) This may be fixed on app level, if it hasnt been yet, but as of beginning of 2012 when I tested the GPS apps, neither of them would get priority over other applications, so if any other app pops a notification (i.e. you receive a text message) while you are riding, focus will be switched away from GPS app, which means the device will dim the screen after the set timeout, and if it does, you'll need to get the screen on, possibly enter unlock code, and click the GPS app icon again. Trying to do all of that while going through turns is no fun.
4) (not really applicable to you, but I figure, I'll include it anyways - phone GPS is not a good combo - if you crash, you are likely to have lost your phone/GPS unit, so you lost your comm.
To elaborate on #2, the major issues I found come in planned routes - the interface of these two apps still leaves some things desired - it takes too many "clicks" to mark a waypoint as visited, for example, and while in a car it's not such a big deal, doing so on motorcycle really makes a good UI stand out.
I also found these apps to be not as precise (due to hardware limitations) and freezing sometimes - also a big deal if you are riding looking for the next turn.
Overall, in my quest for getting my phone to be all-in-one device, I came to conclusion that I need to have a dedicated GPS unit. I ended up getting a decent non-waterproof Garmin unit, and getting a RAM waterproof enclosure for it. Watch out for cheaper units - some of them dont have basic route planning features.
Hope this helps