I say she is still very viable (I main her). In my opinion she's a good counterpick (against skillshotbased longranged burstmages like Lux), but not really a safe pick; she's very easily counterpicked by shortranged pointandclick mages like Ryze.
She still has incredible DPS with her Hate Spike, and Ravage's AS buff lets you take down towers like crazy. Her ulti was always more of a utility skill, though the recent nerf did harm her burst. Good thing she's not a burstmage, she's a DPSmage.
Another thing that the nerf impacted was increasing her Hate Spike mana cost. Spending 40 mana every second isn't cheap; you now need to play a bit smarter with your mana as her.
Also, if the enemy lanes buy pink wards... it is no different from them buying normal wards if you're someone else. Except that pinks are twice as expensive, putting them far behind your build. Getting a couple successive ganks off can make a really snowbally Eve and usually a won game.
Evelynn is also a monster in teamfights. She's similar to the role of Nocturne in teamfights, dealing sustained damage from melee range that just won't stop. But here is where her ulti really shines. She can slow an entire team with it, messing up their positioning, as well as getting herself a large shield to help initiate better. It may make little sense for a squishy assassin to initiate, but with some tank items (*cough cough* Rylai+Warmog's) it can, and indeed should, be done.
All in all, I say that since she's a real cheap champion, I suggest buying her. She's a very strong champion that can deal large amount of sustained damage with a huge amount of mobility. Oh, and the Hate Spike + Dark Frenzy (passive) can make for some amusing failures for the enemy to towerdive you .