My instructors always warn me/our students against roundhouse kicks, just in the chance that the person who is attacking you is an experienced fighter. The reasoning is that the roundhouse telegraphs too much, and there are various counters, and things to do to stop one. So we tend to favor the "wheel-kick" where the knee comes forward, then you switch your hips to deliver. That being said, I think a low roundhouse to the knee or inside/outside of the leg works great in a pinch.