Mastercutor and Aaronrules380 said evrything I love about him (brilliant plan that got ruined only by lucky flukes, his scheming bastardry etc.), while Mastercutor and King Cannon said everything I loathe about him (two words: Impel Down). With that I'll give a score of 4/5, but I'd like to give some extra thoughts on him as a character.
I think he's the anti-Doflamingo.
We probably have two of the most powerful and brilliant Warlords of the Sea heading fearsome organisation ("New Age" crews and Doflamingo organisation for the former, Baroque Works for the latter), and relying on seconday objectives to gain power (Doflamingo and the Riku family takeover + SMILE, Crocodile and the Alabasta takeover +Pluton) but their motives and personality diverge quite a lot.
With Doflamingo, contrary to what we were led to believe in the Jaya Island arc, we had somebody holding onto a form of idealism, that drive to attain his goal of being the Pirate King and we see how it spills into his relationships and leadership: he is very harsh (dunking a guy's head in a barrel, Bellamy hah) but is mindful of how he treats those who have proven loyalty (Monet and Vergo say hi again). In contrast, we have Crocodile, so jaded and cynical, he views those working under him in strictly working conditions (as opposed to Miss Golden Week's cover story revealing some semblance of friendship among the agents) and even with suspicion (sorry Robin).
And then there's how they reacted with Luffy. We all know how it went with Crocodile: sure that guy took a big bruising with the ego but Luffy did spark something within him: a sort of "romance", as the aftermath of the Marineford arc revealed, and I'm sure he ends up more powerful and with idealism to boot because of that. Doflamingo? Besides the Robin mishap, Luffy is causing quite the trouble for him and even though we're just midway through Dressrosa, I have a nagging feeling that our pirate protagonist will play a part in not just reducing Doflamingo's role heavily but also in how he will lose a certain part of his current idealism.