I don’t like them. They don’t flow well and I think make the hand and arm more apt to fatigue. Sometimes the mood strikes me to use one, but I am picky about the make and feel. Usually I opt for a fountain or rollerball.
I like some rollerballs like the Pliot V-Ball and Pilot Precise. I mostly carry rollerballs with me when I’m out for convenience and also risk-minimization. The risks to which I refer are leakage, loss of a pen, someone else using my pen, taking it or ruining it. Many people do not know how to write with fountain pens and therefore practically no one is allowed to use my prized FPs. I will let others try my Pilot Varsity FPs because at $4 a pop with a fairly sturdy steel nib, there is a low risk of harm.
If you don’t already know, high-end gold nibs will conform to your writing style after a while. If you have a gold-nibbed FP, you might want to think twice about letting others use it, as it may influence the nib.