I've bought and own all three kinds of bags (panniers, rack top and saddle) and use them depending on the occasion/mileage. f you don't want to spend spend too much money on something you don't know you'll need, I recommend the following bag bought on eBay for $15 and after 2 1/2 months of almost daily use, still holds up.

http://cgi.ebay.com/ws/eBayISAPI.dll...d=615976444020

FYI: now that I know that this type of bag fits my daily commuting needs, whenever it fails, I will go ahead and invest in something of better quality, not that it's falling apart, but the suggestions above will probably last quite longer.

Hope this helps!