I was a bit wary of it as its Alpha, but it seems stable enough on my rig. There's no appreciable hit to framerates that I can see, or any hit on network performance.
The overlay is unobtrusive (except it lists the proggy title and game title instead of just the names of your fellow TS users) and it does exactly what it says on the tin.
Top program and I reckon we should ALL get it, it'd make those confusing chats much clearer AND when you yell for help, everyone can see who's doing the yelling...