First of all, thanks to Hydrolyphic! This is a great idea for a thread and I liked reading what those above me have already written.
If I was trying to describe them as briefly as possible I would say that they're "classic hip hop in a really funky, charming package." But if someone would let me, I'd say all this so as to describe them as accurately possible:
Beats: You have to mention the beats first because that's really what People Under the Stairs are about. And it shows because the beats are world-class in my opinion. Heavy use of samples, which to me, is a plus. Intricate, well-made, funky and enjoyable.
Rap: Their vocals are well-delivered and charming. The lyrics aren't "world-class" in the same vein as the beats, but they're often very witty and are certainly enjoyable. Furthermore they avoid posing and keep their subject matter both interesting and true to themselves.
Other: They're a really charming group. They have an awesome "flavor" about them; Both Double K and Thes have great personalities. I love their philosophy towards life and their love of music.
Obviously I'm still trying to be concise in the above explanation but I feel that much is needed to do them even partial justice. Looking forward to reading the rest of the responses. Peace!