I use a cheapo one I got on fleabay for like $12 USD. I toss it on my cross race bike when I use it to commute. Drop bars, attaches with three Velcro straps. It's not large, but can fit a wind jacket, wallet, phone, and a few others small things. (I'm lucky, can keep all that I need at my office - clothes, food, eyeglasses, etc.)
I'd say go cheapo first, see if you like it, then upgrade over time. I love mine. When I want to change things up and not ride one of my commuter bikes, I toss the bag on my road or cross bike and I'm good to ride!