Almost the best...
In spite of that limitation, it compares prices perfectly, auto creates a shopping list grouped by stores (using best price per item), and its interface is clean & awesome. Not too many features, it's just right.
If it could be synced for multi-user use, I'd give it 6 stars.
On a minor note, it would also be nice if one could exclude a store from the list if you're simply not going to that store today regardless of price. But that's a minor issue IMO.
Great work!