My MTB has a rear rack & it's great. Weighs hardly anything (made from aluminum). I had to get one that had as much heel strike clearance as I could find (my feet are too big/long).

My German bike didn't have a rack so I got one off a wrecked bike here at the study center and since I have to buy groceries & haul them back to my apt., it has worked great.

At home I have this wonderful trunk bag but here I just use one of those gimme shopping bags that I've gotten at a convention to haul stuff home. Put everything in it then fold over top & strap it down. Bought a flat strap-style bungee with 3 flat straps that does a MUCH better job of holding things in place than a round bungee would.

The Germans use wire baskets, milk crates, all kinds of containers mounted on their rear racks. Since their bikes ARE their cars most days of the week, being able to plop backpack, groceries, lock, etc. into the container on the rear rack is expected. In fact, from thinking about it, only the road racing bikes are sold without rear racks - all other bikes seem to come with them.

The Commuting and Touring BF forums have endless threads about different racks & bags & panniers. As witnessed by how many responses you got to this question, eh?

best,

Tom