I have one and it's nice... but I hardly ever get to use it since the country around here isn't very flat. (IIRC, you live in the same country, in a different Country, right?)
You'll also need to pay close attention to throttle grip installation if you've changed grips from OE, or change later. If you get the grip on a little too far, it won't work.
If you use a good grip glue, a mistake could mean buying another set of grips.
FWIW, for I think the cheaper locks that you latch with a thumb are easier to use (having had one of those on a previous bike)... but they do take up grip space and aren't very good looking- especially considering the transparency of the TM.
I get the most use out of my Throttle Rocker - it allows me to have no grip at all on the bar, operating the throttle with my palm, or I can place my hand out on the large TM bar end and operate it by thumb to give my hand a break. Some folks don't like these little clip on devices and I'll admit, it does take a little getting used to. After a day or so, I was completely sold and use it all the time. Some prefer to rotate them out of the way for most riding and then just rotate them in place when they want to use them.
Too bad more bikes didn't come with REAL cruise control systems. Perhaps with more bikes going to computer controlled EFI, we'll start to see more OE cruise controls?