I've been considering getting a trackball for a while, but there aren't a lot of options (at least not from standard shopping places, and I'm definitely not up for paying an insane amount for a premium device). A thing that has held me back for quite a while is that trackballs don't seem very suitable for gaming... but I don't game much these days, and it wouldn't be that bad digging out a standard mouse when I feel like gaming (especially not if the "Logitech Unifying technology" works and you don't have to plug in anything when adding a new device).
Avoiding carpal tunnel is much more of a concern now than whether the input device is good for gaming. A friend of mine got hit by this recently, and ended up buying a RollerMouse Pro 2
. Seems like a pretty device (and insanely expensive!) nifty, although it takes a bit getting used to - different sensitivity than using a mouse, and you don't have your usual Mouse Muscle Memory. Bonus point for using optical sensors instead of being mechanical, probably also part of the reason why it's not exactly cheap.
The logitech M570 looks very nice, and is something I'll consider if they make a left-handed version, and ends up at a reasonable pricetag (the cheapest I can find it at here in .dk is ~USD100). A trackball still has the disadvantage that you have to move your hand between the trackball and the keyboard; might not be that bad wrt. CTS and strain in general, but it's still annoying. For the RollerMouse, the distance you have to move your hands is much shorter - I haven't used it enough to know whether you end up accidentally moving the mouse pointer around with your wrists, though