I'm also a fan of Sea To Summit; they keep things dry and compressed; I've compressed my down TQ & UQ's for a number of days and it doesn't seem to affect them; keep in mind they were taken out each night, fluffed and used to sleep in. I don't think it would be good for long term storage though but I'm not knowledgeable enough about the long term impact of storing them compressed. But there's simply no need to.
The only down side to the Sea to Summit compression/dry sacks is that they're not light. for vehicle comping not a problem but for lighter backpacking applicatons they're a bit heavy.
If you want water resistant and light weight you can consider these bags from ZPacks:
They're super light weight and will protect down from water; they're not compression sacks but you will be able to "compress" your down as you pack it and make it a pretty small package.