With the last patch (23.06.2011) Evelynn got nerfed hard...
Now, she slows enemys when she attacks them in stealth instead of stunning...
Her Ultimate got nerfed too:
Passive heal reduced to 150/225/300 from 350/500/650
Attack speed bonus reduced to 25/50/75% from 50/75/100%

So... to my way I was thinking She´s worthless...
BUT I tried a new build for her...
NOT AP, but AD!
It works great...
together with her ultimate, she does so awesome damgae...

LAST UPDATE:
25.06.11

Pro´s:
- Awesome damage
- EPIC FAST
- Her ultimate heals her with every dead enemy
- Stealth
- Can slow
- Reduces Magic resist / Armor
- Cool spam-attack
- annoying
- map control

Con´s:
- squichy
- Poor earlygame
- Can be gankd easyly

[*] Doran's Blade -> good starting item... 100 health, 10 damage, 3% lifesteal

[*] Boots of Mobility -> Best boots for "Eve" (my opinion), because if you are unvisible, you have to be fast (You can also take berserker)

[*] Manamune -> Quite nice item... You get much Mana, Manareg and good attack damage (You just need it, because "Eve" wasts so much Mana... This item is your start... from now on you do damage

[*] Frozen Mallet -> As I said, "Eve" is squichy... you need the life-pool... 20 damage are okay, but the important thing is the passive... your enemys can´t be slow enough :D

[*] Infinity Edge -> I don´t think that I have to explain this item... Awesome damage, critchance and an epoic passive (now your damage is gtting really serious) ;)

[*] Phantom Dancer -> Really nice item... much attack-speed, critchange and (really important) walk-speed

[*] The Bloodthirster -> This is just the best poweritem for the last item... much attack-damage and much lifesteal ;)

IMPORTANT! Buy "Oracle's Elixir" whenever you can to see wards of your enemies, which could be the reason, why you get ganked the whole time :D

Ghost -> As I said as "Eve" you can´t be fast enough... But this spell is EVERYTIME important... to escape, to kill enemies and for mapcontrol

Ignite -> Just to support your damage (earlygame)
YOU CAN ALSO USE "FLASH"

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Flash -> "Flash" is in most cases a cool spell... extremely important for escaping... But I don´t think you need it for "Eve"... But if you think so you can take it :D

Teleportation -> Also cool for map-control... but as I said "Eve" is so fast... you can walk trough the map in no time...

[*] Hate Spike (Q) - Hate spike is a really cool spam-skill... its nice to jungle, kill masses of minions or kill even two enemies

[*] Shadow Walk (W) - If you play "Eve" you should ALWAYS be invisible... its a quite nice skill... BUT just pay attation and destroy enemies wards to dont get seen (buy Oracle)

[*] Ravage (E) - This skill is AWESOME! It does much damage in earlygame and reduces magic resist/Armor of the enemy... so its nice to kill enemies or minions fast

[*] Malice and Spite (R) - The passive of this skill is cool in teamfights or if its 2vs1 against you... The active is awesome, too... You can use it to destroy buildings (backdoor :D), to fight against/follow enemies or to escape.

[*] Determined Killer (Passive) Its a cool passiv in earlygame, but I thinks it´s useless...

There are many "Eve"-Jungle builds on Mobafire.com... if you want to jungle in beginning use them... I´m not a fan of jungling at all... but in mid-game blue- and redbuff could be a good choice and in lategame, it´s a MUSTHAVE...
By the way... fullequiped you can take down "BARON" easily alone...