Beginners Guide to riding Fixed/Single Speed in London

Sigh...
Like i've said before...''If you're gonna speak for a whole generation....you better know enough to handle it's education''

The running backpacks you hold so dear are designed for....running.
Cycling specific bags are different.
The backpacks you recommend AREN'T 100% waterproof...and they AREN'T tough enough or big enough to handle a daily commute if you're the kind of person that commutes with a laptop,spare shoes,clothing,tools...etc
There is a reason that messenger style bags and backpacks are so popular...and it isn't JUST because they're seen as cool.
They're made from very strong cordura...the best have extra reinforcing....better padded backs and straps...floating liners to make them waterproof rather than simply relying on a single layer of fabric that the manufacturer claims is waterproof...but after a few months of having things scraping against it actually isn't. They're big enough to fit plenty in...a lot of people do their shopping after work so need a big bag to fit that in too...and again the best ones can be compressed or expanded depending on how much is in them.
They're quick to adjust and access without having to completely take them off.

I said before that calling them Gay was a naive response...and i stand by that.
So long as you're happy with your running backpacks that's fine....but recommending other beginners use a bag designed for a different purpose when there are far better options out their isn't so helpful....and then claiming that another reason they're not good is cos they're too expensive is again very naive.
A good bag will outlast most peoples bikes...and plenty of people....beginners included seem happy to splash large amounts of cash on H+ son rims and other in vogue products. As is often the case...you tend to get what you pay for.

Sorry for the rant....but as a bagmaker i get a bit annoyed when people who clearly don't understand why a good bag is a good purchase start criticizing things they clearly don't understand.

One word about inno-v8 though: THey are incredibly tough despite their lightweight. And although you are right they are not totally waterproof, you can always put something like this inside it:

http://www.outdoorscotland.co.uk/shop/accessories-dry-bags-tpu.htm

My biggest problem with messenger bags if seeing how you can do decent speed on them up big hills, etc because of their awkward shape (not that I have tried so perhaps I should).