hi, i'm wondering if anyone knows of any good product that functions well as both a pannier bag/rack top bag and as a backpack/messenger bag? thanks.

Backpack + bungee cord or basket.

All the backpack/pannier combos I have seen have been terrible backpacks and terrible panniers.

I don't know from personal experience, but Ortlieb has some pannier/messenger bag combos you could look into. Arkel has some pannier/backpack ones. I have an el cheapo one from Performance, which I got on sale for like 40...it's fine as both, but probably not of the highest quality or durability. Also not the most comfortable as a messenger bag.

How much weight are you looking to haul? If it's not too much, then you might check out the klickfix system of accessories.