"Appropriately" fast greens are a blast. Excessively fast or slow greens not so much.
Take (for example) The Masters: ANGC has some wildly sloped greens, designed to reward the player who hits it in the right, and sometimes small and tight, spot. In the event, greens are wicked fast...to the point even the best players in the world can look silly. The average player would have a hard time with this combination. Thankfully, these green speeds are for the event, and the green speeds are backed down for the members.
I don't enjoy slow greens, ever. For me, anything below a 9 to 9-1/2 is both boring and frustrating. If greens have to be slow due to excessive slopes, the game isn't fun. Imagine The Masters at an 8 stimp...yawn.
Balance is the key.