GrahamG said:
Now the panniers are getting their praises sung by the land-of-the-flat Londoners only thus far - my perspective on this is that if you have a hilly route and regularly get out the saddle to honk up these bits rather than purely using your gears then you really won't appreciate having the weight on the bike.
Don't get me wrong, for touring or for longer distances (say 8-10+ miles?) they're a no brainer but personally I'd rather have my bike nice and light and the bag on me so I can honk up hills with the bike getting thrown side to side and also carry the bike up and down stairs at stations etc. without crippling my arm thanks to the extra unbalanced weight.
Third thing to consider though is sweaty back - if you change anyway then fine but otherwise this goes in their favour.
This post is truly bonjworthy, you have clearly never ridden with panniers in your life. What difference does it make if the weight is attached to you or the bike for going uphill? This is sheer crazy logic.
I also have a decent hill to tackle on my oh so flat seven mile London commute and regularly get out of the saddle. When I do I am very glad that my gear has a low centre of gravity and is nice and steady, firmly attached to the bike, rather than swinging wildly about on my back.
You see, unlike you I have made extensive use of both rucksacks, messenger bags and panniers and can say with that authority that panniers win on at least seven out of ten counts.
I will give that they can be more cumbersome to carry around when you leave the bike. You cannot look like a non cyclist carrying around a pair of panniers either.
But another plus is that they are completely waterproof without having to pull on extra covers and that they give your bike significant extra width ensuring that you are given a slightly wider berth.