My old StreetPilot III has been banged around on two different bikes for almost two years, now. It has been all over the continental U.S., western Canada, Alaska and to Labrador and back. I love it! But, the newer Garmin SP 2720 has a much faster processor, has all the maps for the U.S. and Canada built-in, is more user friendly... Time marches on.
Here is Garmin's page re StreetPilot 2720:
http://www.garmin.com/products/sp2720/
Here is a PC Magazine review (Editor's Choice winner) of the 2720:
http://www.pcmag.com/article2/0,1759,1866425,00.asp
On my R1150RT, I used a mount from GadgetGuy. It is unobtrusive but rock solid. I think you could pick up the bike by the GadgetGuy GPS mount! His stuff is not inexpensive, but elegant. I will probably use his handle bar mount on the ST4s. This may necessitate a change/compromise as to tank bag choice. Just have to see what works best when I have the bike and GPS together in the same spot. On my F650GS, I use a Touratech mount and it works fine. Don't know if they have anything for the Duc. Might check out that possibility, too.
Here is picture on the GadgetGuy site of SP III mounted on an ST4:
http://www.gadgetguy.net/PhotoGallery/DucatiST4.html
You will also find a picture on that site of an SP III mounted on a Duc using a Touratech mount on a GadgetGuy base. Seems like needless expense, but that may get the GPS up above the tank bag. Again, I'll just have to see.
By the way, the 2720 is a bit smaller than the SP III pictured.
Obviously, you have to get power to the unit. I hardwire it so that it is always on. It draws very little juice and there are times you want to use it when the bike is not running. Also, it keeps running statistics (total time, moving time, stopped time...) that I like to have from first start-up in the morning to last shut down at night when traveling. YMMV.
If you want to be able to hear the voice prompts, you have to tie it into your sound system, if any, or use ear plugs direct to the unit. The voice prompts are helpful, especially when you are in traffic in an unfamiliar place and are concentrating on your environment and have minimal resources left for navigation, but certainly not necessary.
The PC Magazine piece has a link to places to buy and prices. Though the MSRP is $1076.91 (Weird!), the unit is readily available for under $800. If I could have only one after-market accessory on a motorcycle it would definitely be GPS.
Hope this helps a little. Good luck!