I had a report from a pilot once having a near miss with one, he seemed pretty shaken as it only entered his field of vision at the closest point of 'near' impact.
When they compared the ATC radar against the Met dept tracking of the balloon launched some 15NM away they couldn't tell them apart... Big sky theory and all...
I would guess that any impact would be very minimal they are only latex and the transponder/reflector would weigh well under 1kg and are mostly hollow so would 'crumple' on impact. Still things struck in the wrong way can do amazing damage, ie moderately large birds...
Denti, I believe they do both in various locations, but
AFAIK the balloons never show up on ATC scopes.