I also use the Topeak handlebar bag. I like it because it's small enough to fit on my small handlebars (38cm) without getting in the way. I use the additional mount bar that it comes with for my HRM/bike computer and it works really well.
Like velogirl mentioned, it's a great size because even packed full, it doesn't affect the balance and steering.
I used it on my century last month and can verify that the rain cover works!![]()