I've found one thing with Silly Putty that I don't like. I tend to use enamel paints and it 'softens' the putty quite a bit. There's various degrees here and it's not a show-stopper, just something to be aware of and watch out for.
I really like using Silly Putty for soft-edge masking. Works very well for what I've done. I found that when I do a hard-edge with it, the paint builds up at the edge and creates a ridge in the paint that needs to be sanded down after the paint dries. Tape does this too, but not to the same extent that I saw with the Silly Putty.
Filling holes and stuff works well too (as mentioned). I also have some rubber cement, but haven't been able to try it out. I actually was going ot use it for 'build-ups' (ie. assembling pieces to check fit prior to final glue and paint.), but masking sounds like a good idea too.
Murray