The puck is only a GPS coordinate receiver. It simply receives signals from the satellites in order to plot your location (via lattitude/longitude) on a map. The maps still come from the data plan--not the puck.
I've connected my Freedom gps to my BB 8830 in its Otterbox using a lanyard. My keychain is already crowded with my car's clicker and my garage clicker. While the keys generally go with me, not all the time. The other day we did a bike ride from a friend's cabin and I had no need for my keys but the BB tracked the 40 mile ride using the puck.
I just purchased one of these, and i must say, if i don't have it connected to my keys and it's in my pocked, it's light enough and small enough i dont even know it's there.... It is alittle pricey, but it is no doubt worth the money.... you can't go wrong if you pick one up!
Even with car keys, it's not that bad... unless you have a janitors(read: alot of keys) type keyring then you wont hardly notice it... but i guess if you have that many keys, the puck would be unnoticable anyways....