How well do silicone sprays like Scotchguard help to keep the water off of your clothing.

I have several nylon/ cotton jackets that I would like to make waterproof.

My experience with Scotchguard has been mixed – on synthetic fabrics with a tight weave, like nylon windbreakers, it seems to work fine but requires at least an annual reapplication, and definitely after every washing. On natural fabrics like cotton, I have found that it slows down the absorption of water/snow/sleet, but does not prevent it. If you are outside in a drenching downpour though, forget it – Scotchguard doesn’t work worth a damn at waterproofing, its strength is making things water resistant.
Again, these are just my experiences, applying the product according to manufacturer’s instructions.
One caution – do NOT apply this stuff in an enclosed area, like inside your house, especially in the basement or where there may be an ignition source like a pilot light. Best to do it on an outside porch, or in a garage with the door completely open. Then allow the article to dry COMPLETELY and even let it hang out an extra day to air out. The chemicals in these spray waterproofing compounds can do some not very nice things to your lungs if inhaled while they are still wet or drying.