I'd be interested in hearing other peoples' experiences too. I've tried both flask-restless and flask-restful previously but didn't get along with either of them.
I recently created a new API and went for a very simple flask.Methodview along with Marshmallow [0] for (de)serialization. I then wrote a simple replacement for marshmallow-alchemy since it wouldn't do object loading without a lot of flapping about.
I found Marshmallow really nice to work with (I'd been using Schematics in the past).