Poplar is an excellent choice for paneling walls. It's lightweight and accepts stain or paint exceptionally well. The lack of pine tar makes it easy to work with and cleaner. I'd seal it with a sanding sealer, or stain, or polyurethane if you want a cleanable moisture resistant finish. If you have bare studs, you might consider using a black plastic for a vapor barrier. That will make it look better at the joints. Installing it on the diagonal looks nice too!
Yep, very rustic and natural. Sounds good to me.