i did a post on sigg bottles a couple of months ago (here), and have since bought one (0.6l, in red). i think the bottle itself is great, and my reasons can be found on the original post. my reason for writing another post is about the screw top cap, and what a great design it is.
not only is it comfortable to screw on and off, but the hole is so multifunctional! it's great for attaching to a bag strap, or securing with a bungee cord to something. but possibly the best part is how comfortable it is to hold with just one finger through it; great!