Here's a Hummingbird that has a phenolic riser. Also have a Bob Lee that has it in the riser as well, and I quite like the heft of both- not to mention the aesthetics. Yes, it does add weight to the bow, but not 'a ton'. Can't give an accurate answer, but on average, I would say no more than about 30% increase, and this is only when it is used in a substantial manner as in this bow.
The extra heft and mass give me a sense of greater stability. I get the same thing from some of my bows that have alot of ebony in them. I'm largely a 1 pc recurve shooter, and prefer a bow with a bit more weight. I find the combination of a good realease and the solidity of a slightly heavier riser to be a very effective combination that works nicely for me.