I know how to draw landscapes/3D drawings accurately since I was 11. (13 now) It's fun once you get the hang of it but I somehow instantly started drawing in 3D pretty easily when I was younger. Strange really. Idea: Once your game is compatible with texture packs: I'm making a pixel pack for people who want higher FPS while adding a Minecraft feel to the game.
P.S. Tablets are hard to use for beginners >.<
Landscapes are what I do best. I have no problem with perspective in general but figure drawing in perspective always gave me trouble. Spending 20 minutes trying to make the forearm not look twisted, etc..
I have a real live paper sketch book that I use for concept noodling because somehow having a real pencil and textured paper in my hand is more freeing. Though it's taken me a while to get back into that, also.
I've had digital tablets over the years of various kinds but it's been at least 10 years since I used one with any consistency.
Last year for my birthday I got myself a large one with nice buttons, etc... and I've been working on getting comfortable with it again. The picture above is the first real sketching I've done on it though. The size helps... and I finally got the sensitivity to where I like it. When I first got it, I hadn't sketched in so long that I didn't know what feel it should even have.
The size is nice because it pretty much forces me to pick it up and hold it like a sketch book. The buttons are conveniently at my thumb (including the Undo button I setup

) and sketching that way feels much more natural.
For the curious, this is what I have:
http://www.vistablet.net/products/vtmuse.html I didn't pay that much for it, though, as I bought it on Amazon.