103...From experience...I have one that is either externally or internally powered,Garmin model 76SCX...it is great, but does not compare in features to the 2820 I now have. I still use the 76, as it is more of a "hand held" type unit, but generally for purposes other than the bike. You will find that once you get this little item, you will use it MUCH more than you ever thought you would. It is useful to find places to eat, gas stations, and other POI's (points of interest), as others have mentioned above. It is also great to get a view of the road ahead, so you know what to expect.
Once you get this, you will wonder how you ever lived without it...also very useful in the car!!
Take a look at the other threads below, for numerous reviews, mounting options, power options, etc..