I had the opportunity to do an Outside Broadcast with a NEW digital OB Truck with ALL new gear.
The shotgun microphones had this as a cover in the kits.....
They work great... after 8 hrs of med/heavy rain the mics and blimps were were dry.
Previously i have tried many ways of keeping the mics dry from thin plastic bags to no cover....nothing with huge success. http://www.rycote.com/products/accessories/duck/
ps.Im not a salesman only a freelance audio guy