These guys are pretty much what you'd expect: monkeys with wings. They can be vicious, like when they "caught the Scarecrow, and with their long fingers pulled all of the straw out of his clothes and head" (12.52), and they can be considerate, like when "carefully and gently, they lifted Dorothy in their arms and carried her swiftly through the air" (12.56). They don't have much choice about how they behave; they're obligated to work for whomever has the Golden Cap, which was enchanted by a powerful sorceress years ago.
As a result, the Winged Monkeys are neither good guys nor bad guys. They just do what they're told. Over the course of the novel, they serve both the Wicked Witch of the West and Dorothy. But just because they're enslaved doesn't mean they're not having fun. It seems like these guys are always chattering and tittering. In fact, it was playfulness that got them into trouble with the aforementioned sorceress in the first place. The current Monkey King's grandfather, who "loved a joke better than a good dinner" (14.25), played a prank on her betrothed, and he and his ancestors have been paying for it for a long time.
Thankfully, once Glinda makes her three requests of the monkeys, she returns the cap to the King so that "he and his band may be free for evermore" (23.21).