Beginners Guide to riding Fixed/Single Speed in London

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:

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).

Your bag may seem incredibly tough to you but as i said....unless you're carrying your laptop,shoes,tools,a few days worth of shopping in it on a regular basis, then it's really not going through any abuse at all.
You CAN'T fit all the stuff i mentioned into a running backpack no matter how you dress it up.
As for awkward shape...you should DEFINATELY try riding some distance with one on both empty and fully loaded.....both myself and most other bagmakers go to a lot of trouble to make their bags fit very comfortably...especially when you have to carry very heavy loads.
Both myself and everybody else i know who uses a decent bag has no problem climbing hills wearing a good messenger bag.
Maybe it's your hillclimbing technique at fault here?
A top quality messenger bag has the right fit and the right stabilizing/x-straps to stop swinging and unwanted movement.
No matter what backpack you have...if it has the equivalent weight in it as a messenger bag, it will always suffer from bounce on potholes,kerbs,road bumps...whatever.

As is always the case...if you haven't actually tested something extensively...you should leave it to those that have to talk about the pros and cons of said product.

As for panniers...why would anybody want to have to keep removing and refitting them everytime they have to lock their bikes up...go to tescos...etc when they can just use a good quality messenger bag or backpack.

If you're more interested in bags for riding long distance rides and hillclimbs...don't go claiming those bags are a better option for commuters...when you're talking about two completely different situations.