1. Retargeting is awesome for AMMs, though it does make it so that you basically have to use sizes larger than 1MSP. I did 2MSP missiles. Works pretty well.
As far as ASM retargeting, I put it on mine and found it to be useful because even with good speed, the hit chances aren't amazing. Yes the PD gets another chance to shoot it down, but I was putting 2 decoys on my ASMs with ECM anyway so I think it works out pretty well.