not particularly. They are good for GPP and for newbies, but they won't bulk you up unless you can find a way to add weight to your pushups every week.
hell no, maybe a little tone, but just doing your weight alone won't build crap. I'm just going by how my buddies from the military look. very strong but not big
I think it could be good if you were trying to burn the muscles. for example do a set on the bench press and immediately get on the floor and do pushups.