I think AP Rek'Sai is more of a high risk build; you really, really need items to be relevant. Her AP ratio on Q is really high, but the base damage is very low, thus you really have to sacrifice a lot as far as her build and the rest of her potential to get her to the point where she's doing that sort of damage. Sort of to put it in perspective; Rek'Sai with 6 items is doing about the same damage as Nidalee with 3/3.5. It's still good, don't get me wrong, and Rek'Sai has some advantages over Nidalee like her Q not costing mana, being AoE (not huge, sort of like Corki rockets), and being a faster skillshot thus harder to dodge, but the fact that Rek'Sai loses a lot of her potential from just not being tanky enough is a pretty big downside. It's great for sieging and can be useful if you have other teammates tanking and you can just sit back, kite around, and just poke people down, but if you're playing jungler you're more likely going to need to be doing more to peel for your actual APC to do damage.
I still think it's fun and fairly powerful and I'll probably still be playing it occasionally, but it's not something I'd bring into ranked.