I have one on my Bandit, and I think it's really nice to have. The probe gets the oil as a result of the prodigious splashing/slinging of oil off the spinning clutch (try running your engine w/o the filler cap removed one day). Purists would say that it's got to be inaccurate compared to the temps as the oil is entering the cooler. Maybe so, but who cares? The purpose of the gauge is to show a general range, and more importantly, if there's a change in what you normally run at. Oh and I got mine in Farenheit, not that it matters one bit.
However, I had not considered putting one on the FJR as it already has a temperature gauge. The Bandit is oil/air cooled and has no form of temperature gauge.