I am by no means a pro, or close to it. I am actually still kinda new to the game, so I'm sorry if I get stuff wrong, or whatever.
I've seen around that elise is terrible and so on and so fourth. I have been playing around with a couple of different builds with her. Obviously everyone will like their own play style, I found that I like her on AD. Especially in twisted treeline. I rarely end up using her human form. Really only as a means of escape so I can use the stun. By the end of this build I was able to dominate the game. It was also fairly easy to survive. My other two teammates had been disconnecting throughout the whole game and we still managed to win because of Elise.
-I started with boots and 3 potions and went to HoG/Brutilizer first, then moving into Lord Van Damm's Awesomesauce.
-For boots I with with CDR, because I think it helps the most.
-Next I went with Phantom Dancer to get the increased attack speed, crit, and movement speed. This will make your spider W insanely fast.
-After that I went with IE, obviously to dish out tons of damage, and the increased crit damage goes well with PD.
-I wanted a little more survivability so I next went with Blade of the Ruined King. I like the movement speed it gives you, and the heal can go nice with spider W.
-Lastly I went with Overlord's Bloodmail to again increase my sustainability. Like I said before, I mainly used spider form which doesnt use mana. So with all the increased lifesteal and healing I almost never had to go back.
This makes your human form pretty weak. I really only use it to get my spiderlings back and for the stun and her Q since it is % of health based. Even though it's not much E>Q>R>W>Q usually took them out before they had time to react.
Remember, that your E in spider mode isn't only a means of catching up. You can use it to escape by staying next to walls and jumping to minions on the other side. You can also use it to get rid of turret aggro.
Your spiderlings are actually a nice chunk of your damage. Make sure you keep them up as much as you can.
I'm not level 30 or anything so I don't have any runes, and I went with what points I do have for masteries in the AD side of the offensive tree.
For those of you who want to see it in action, it's on lolreplay the link is below. I know I am not the best at this game by any sort. I have a lot to improve on and I am sure the people I was against weren't the smartest either.. but it's just meant to be an example or a gateway to better builds.
I also realize this build is kinda a twisted treeline only type thing, but I am sure it could be easily modified for summoners rift. It's also lacking armor and MR, and that's where bloodmail would be traded out for something. Again, just an example take and use it how you will.