...and I've used it more.
My first impression was that it wasn't anything that my Kensington didn't offer. After using it for a while and going back to the Kensington I realized that I was completely wrong.
The steel rollers on the CST alone make it worth the extra over the Kensington. Despite regular cleaning, the Kensington just gunks itself up too easily, after a few months of use the dirt really gets itself inside the rollers and is practically impossible to remove. I'm sure if I disassembled it and cleaned it out it would go away, but it seems pretty stupid that I have to do that after a few months, especially given that I keep my stuff clean and don't use it with greasy hands or whatever.
As a consequence, the ball in the KEM feels like it's suspended in treacle. And if you roll it too fast, or try and make particularly fine movements, it doesn't roll properly and the ball jumps, a problem exasperated by the fact that the ball is not secured in place. In the CST, the ball just rolls smoothly, and after several weeks of use, I still haven't had to clean it yet. It also feels far more responsive, and the ability to change sensitivity is great.
In terms of ergonomics, the CST wins again. Obviously YMMV, and bigpook found that the CST caused him agony to use, but here's what I think - in the KEM, the ball sits up quite high which means that while the wrist rest is good for your... wrist, you have to use it with your fingers outstreched quite high. In the CST, the shape of it allows your hand to lie on it which gives better support. If you want to move your hand to reach the scroll wheel, or to get a different grip on the ball, you can just slide your hand up and down and you still get the same effect, whereas the KEM's wrist rest is only really comfortable if the base of your hand is resting at the bottom of the 'hump' on it.
The CST destroys the KEM in terms of build quality. It's made from tough ABS plastic and has the single thickest cord I've ever seen attached to a pointing device. The KEM feels cheap and plasticky by comparison. One thing I'll say is that the CST has a sort of "Unicomp finish" to it - it's very durable, but there's plenty of dimples and flashes across the case. The shiny black ball tends to show up dirt more than the speckled grey one of the KEM.
In the CST the ball is permanently fixed into place. One of the problems I had with the KEM was that it used to fall out if you were carrying it in a bag or whatever. Also, the KEM's wrist rest is attached by very flimsy contacts... I used to be quite paranoid when leaving in a bag lest it bended too much and snapped. The CST doesn't have this issue at all. Of course, in order to clean it, you have to dismantle it, but this is nowhere as difficult as dismantling the KEM - you remove three Philips screws and it just opens up.
In terms of features, the Kensington has the scrollwheel, which is pretty nifty, and the software supports various features. Of course, Kensington doesn't support the thing any more and you're stuck with drivers that only work under XP or Mac OS X up to 10.4. There are third party drivers that support some of the features, but that kinda defeats the point.
The CST has a scroll 'barrel', the ability to switch between 400, 800 and 1600 DPI, and three buttons by default. There are versions that support either two jacks to add a fourth or fifth button, or three jacks to allow you to duplicate the functionality of buttons 1-3, designed for people with various disabilities. In fact, the contact pads for these are included on the PCB irrespective of which version you buy, so if you're into modding, you can just buy the three button model and hack away. There is supposedly some software for the CST, but I'm sure it's just a basic button re-mapper... All the features are built into the hardware already.
In terms of the scroll wheel - both have excellent designs. The main distinction here is that the KEM has a ratchety scroll wheel, and the CST has a free rolling one, like in some of Logitech's higher end mice. For the internet, and reading documents, the free rolling one may be more desirable, but for playing games, the lack of clicking is a bit of a pain if you're switching between weapons or whatever. Some have complained that the KEM's scroll ring feels cheap and is noisy, but I think this is overexaggerated.
