Greg Bender
http://www.thisoldtractor.com/gtbender
The CrampBuster is a well designed product that works. It stays in place and does not rotate or move when used (this is in stark contrast to the "Throttle Rocker", which is of very poor design - see review on this page). It even works well on the curved and slick Gran Tourismo grips.
The CrampBuster is available in four sizes: a narrow and a wide for regular hand grips (1 1/8" diameter) and a narrow and a wide for oversized hand grips (1 3/8" diameter). The wide version will definitely provide a lot more surface area on which to rest you hand. But, it does take up more hand grip width. Personally, I prefered the narrow version because my grips are not very wide. For wider hand grips, the wide version would be great.
As with any change to your handlebar controls, it does take a bit of getting used to. Out on the highway, it is easy to get it adjusted to exactly where you want it to be. Around town, however, you may find that it gets in your way as you do a lot of throttle twisting. But it is quick and easy to take it on and off as desired. Storage is simple: stick it in your pocket, hook it over your belt, wrap it around it an empty spot on your handlebars, stuff it in your tank bag or panniers, etc.
NOTE: The CrampBuster is an original, patented design. The "Throttle Rocker" was copied from the original CrampBuster design and infringed on the CrampBuster patent.