I've never heard an aluminum, or any other metal domed tweeter that sounded "right" to me. I almost always end up with silk or kevlar now. The metal ones just have an edge to them that annoys me. I haven't heard that many of the insanely priced systems using them lately though, so it might be different on those. But since I can't afford them anyway, it's not that big a deal.
The cabinets on Thiels are awesome though, sounds like they are made of granite when you rap on them. A friend had a pair years ago, and they sounded ok, not worth anything near what they cost though.