My first Kensington wireless presenter was the red LED type, similar to the current Si600. When it got lost somewhere in my wanderings, I had no hesitation in replacing it with another Kensington.
The simple, uncluttered design of the new model is similar to the old one. Everything that you actually want to be bothered with during a presentation is conveniently in reach of your thumb, and can be found with confidence without needing to look down. The green laser is a big plus - as others have commented, it is really bright and clear even from a distance. I'm only partly convinced about the "ergonomic" design of the casing - it is bulkier than my old one, which means it looks decidedly odd when dropped into a shirt pocket during a board scribbling moment.
Something that has not been mentioned in other reviews is that as far as the PC is concerned, the presenter is just a mouse. It therefore needs no special drivers, and on the occasions where you have to use a PC that is available in the conference room it is a major plus not to have to install software. There is a small drawback though, which is that when using dual-screen mode in PowerPoint (something I always do to prevent the audience seeing the chaos on my desktop) you have to remember to give one mouse click in the projector screen before the presenter will advance the slides. This is not a big deal, though mildly irritating when you forget, and may be solved in PowerPoint 2010 which I have so far resisted.