So I've been thinking about trying Evelynn lately (even though her winrate is not-so-great lately). I really like her concept, but I have no idea who to play her other than be generally sneaky. What is her optimal build right now? Can I do AP or AD? Should I be making her a straight tank with cinderhulk?
Teach me the way, Eve mains.
I play her as AD.
I get Devourer enchantment, BotRK, TriForce, Mobility Boots, and the last two items being any of Randuins, Maw, Phantom Dancer, or GA.
Spam Q as much as possible. Know that stealth wards will not reveal you when you're invisible. I start games by finding where their jungler is starting. If he's starting Gromp, I sneak in and get as close as I can without revealing myself. Wait for him to get gromp down to my smite range go in and smite it. I get level two and the enemy jungler gets nothing. Depending on who it is, you might even be able to kill them after that.
Her optimal build depends on what you are doing. In general with a team of randoms who you have not had time to make plans with it is safest to build for team fights which means AD bruiser. 
are all items to consider. She can use a lot of other items well though, and part of mastering her is learning to adapt your build to the current game. Even building mostly AD items there is little difference between the AD and AP jungle enchantment.
Full AD or AP builds work great if played right by both you and your team, but die very easy before doing anything if you mess up. Wait on trying such builds till you are more familiar with how to play her in general.
Tank is problematic because other than damage she has no threats built into her kit to make her worth targeting by the other team. If you go tank and get ignored, you do nothing.
Whatever you do, avoid 
unless you really need to map pressure or you are to flank and initiate fights for your team. In the case where the latter is needed, your team is already pretty screwed whatever you do.
Being successful with her is all about predicting what the enemy will do, and being ready to punish them for it.
Do watch replays at http://www.lolking.net/replays/
