Pretty much agree with Battersea. Handy for short periods at best but well worth the money.
I did have a big scare once on the highway. Had it set for some time when a car suddenly changed lanes across my path. I was going around 120km/h. Grabbed the front brake instinctively, front wheel locked and rear kept driving. Front started to slide out and I was very close to a nasty fall but was able to release the brake, back off the throttle and grab the brake again. Since then I only use it for short periods to give my hand a rest or adjust a zip or something like that. They are inherently dangerous if used as a defacto cruise control rather than a temporary throttle lock.