After a few weeks now I've finally hit Mastery 5 on Viego and have to say he's incredibly fun and cerebral. I often think of myself as a jack of all champs master of none but with Viego's ability to temporarily take the form and abilities of his downed opponents I feel like my broad investment across all League champs can finally pay off.
Trying to dance around fights and finding the right opening, then assuming you get a kill trying to re-evaluate the team-fight puzzle and determine whether you should use that champions kit or your own is so incredibly dynamic. Still have a lot to learn, but so far an absolute joy to play.