I'm looking at bags, the S bag looks to fit the bill in terms of size, compartments etc. How water resistant is it? With and without the cover?
On the basis that the C bag is made of the same stuff, I'd say it will stand up to standard drizzle or short showers just fine. Hours of proper rain will eventually soak it, but are you likely to be riding it for hours? I certainly haven't managed to get it wet enough to reach the contents of anything other than the zip pocket of the top flap (which I cunningly store the showercap in, unfolded, to protect the rest of the bag even if I forget to fit it).
The shower cap appears to work. It suffers from the usual flaw that unless it's pissing it down when you set off you won't remember to put it on until too late to keep the bag dry, and it impedes easy access to the pockets. Fitting the cap to the bag while it's on the bike is a bit of a faff, but this may be easier with the S bag.
At Jurek's recommendation, I've just given mine a treatment with Scotchgard to increase the water-repellance. It's yet to be rained on, though.