I picked up the T400 for a friend's PC that I upgraded to win8, and it works great. I was going to get the T620 as it has gesture support until it dawned on me: It supports gestures, but is not a touch screen. This means that I can activate the charms menu from the mouse, but I still have to move the mouse over to the right in order to USE the charms menu. Same for the other edge swipes. So in other words, other than for pich to zoom, the rest of the gestures are mostly useless. the T650 seems equally useless, and most users would be much better off getting a wacom tablet instead.
The T400 has a great feel, and the glass scrolling area is great. You can scroll up, down, left and right, and in theory you could pinch to zoom on it, but that would be too akward. Then it has a top-center button that is the start screen (works so well I changed that on my mouse), and a bottom-center button that is your normal center click (but we assigned it to the paste function). It gets 5 stars in my book.