Is there public transport to the area from the terminals?
Not that I am aware of, no. There is a bus going from a nearby town that actually stops right beneath this spotting area, but this bus don't stop at any terminal (to my knowledge it takes the way through the roundabout), you'd have to walk a bit from the terminal to get to the proper bus stop. The local taxi drivers would surely give you a ride to this bus stop.
And yes it can be a bit frisky up there.. but you have an excellent view AND parking. It's like a big pile of boulders with a flat area on top.
Affirmative on BMA! It can actually be more satisfying to watch planes from BMA since they have a great variety of models - everything from old rugged Antonovs and Ilyushins freight models to the latest in business jets.
The most exciting thing that comes to ARN is Air Force One (and then you aren't allowed anywhere near the airport for several hours anyway) and some SAAB 2000. (My Swedish heart goes out for this nice littl'un though....) Everything else is just "A320, A320, B737, B737, A340, B747, B737, B737"... well you get the picture....
Something that is HIGH on my NEED-TO-SEE-list is an An-225. Now that Concorde isn't gracing the skies anymore, I'll have to make do with the biggest bird there is.
Anyone seeing this, please message me when you hear of an An-225 coming to Sweden. Doesn't matter where, really.