Got to play around with her to get a feel for her skills, but had to use a StupidBot game to do it. Couldn't react fast enough to play against the more cheaty variety.
She's hella fun to play, and really flowy, if that was a word. And much more melee DPS than Tanky DPS, what with her low-ish base health (Under 2k at 18). Also, slightly limited by the fact that there's shit for choice for CD items on her. Farms like a Boss, though.
I'm thinking for her Core:
Shoes of Appropriateness. Generally Mercs, unless it looks like the game is going to be feeding time for your team, or they have no CC. In that case GET IONIANS.
Black Cleaver. She has a
crappy attack speed without it.
Some other CD item. Maybe Ghostblade, though it's kinda crappy.
Also:
TRIFORCE. Maybe. I haven't played enough to see if it's a noticeable boost to damage, but it feels like it should be. if you get your rhythm going*, you can go Q, Proc, normal attack, Q, Proc, attack, Q, Proc, attack. Forever.
*And have decent CD.
Also also, there's a weird bug with her W that delays her auto-attack for the same time as the stun lasts on the enemy. You can get around it by Q-ing something immediately after using it.
EDIT the 2nd:
If you build an item with mana on it, Riven gets a mana bar.