Cycling to Work

I'll be joining the cycling to work brigade soon and knowing that a few do it from the forum I thought I'd ask for some advise.

The bike has a rack and I've been looking on the net for an appropriate bag to use to carry laptop and notepad and a couple other bits.

The laptop specific bags are way expensive though so any other suggestions of panniers that will fit a 15.4" laptop? What do you use?

Thanks

Obvious I know but remember that it will get covered in a lot of road grime and very wet so make sure it is REALLY REALLY water proof. i don't personally carry a laptop so can't help. You can buy small locking top box style affairs that would no doubt fullfil the roll but maybe a bit of overkill.

EDIT - I just use bungees to hold all my bags on the back of the bike. I find them more versatile than bike panniers at they are just normal bags when you need to carry them on foot, and don't restrict you WRT size of what you can carry.