Elise is actually quite strong, the problem is that people seem to have no idea ho to build her. And to be fair it's not very intuitive. She seems to hardcounter a lot of popular tops, I've found - and recently I've been jungling her and having a great time! She's an amazing ganker, and she functions well with few items.
Elise is a great top and she absolutely destroys bruisers in particular. Her laning phase is really, really strong and once she's on someone she's good at making sure they can't escape. Unfortunately, her only "reliable" escape is her spider's E, which isn't always reliable at all as it can only target enemies and hopping into the air for a second will quite often just get you killed if you're not careful.
Using it to teleport to a jungle creep and then run (or flash over a wall) is a really strong way to use it in an escape though. She's a beast at juking because of this.
In the jungle, she's also really good imo. While she's fragile for the first ten minutes or so, the jungle itself poses no real threat to her and she can clear without even using all her pots if you get a half-decent leash on blue. Her ganks, as I mentioned earlier, are godlike. If your laners aren't asleep picking up early kills is a breeze, though she doesn't quite have the burst to kill people by herself early on. Still, they're not going to be getting away without blowing flash and all their disengages. She's not particularly prone to counterganks because she can scout with her W if she feels someone might be waiting to ambush her and in the jungle there are usually plenty of things to E away to.
The one thing that annoys me about her is that her attackspeed is god-awful and so I choose to run attackspeed reds to get the most out of spider W and her spiderform on-hit effect.
Anyway, I mentioned her best builds aren't intuitive - Elise on the surface might look to be a typical mage but going full ap glasscannon is a huge mistake a lot of people fall for. Because her W is the only skill that has remotely good AP scaling (and it can be avoided), it's a lot more beneficial to build like an ap bruiser prioritizing spell penetration. Almost like CDRwick actually.
****, I've even had success with on-hit items. But make sure you have a good amount of beef, because to do the maximum amount of damage Elise has to get in melee and stay there for a bit. Her W is good poke - but usually you'll want to use it as part of your kill combo (If you W in human form, then quickly change into spider and use Q, your explosive spiderling will also jump onto your target, saving you from having to steer it around minions and tanks) and by itself it's just not enough damage to be meaningful in a teamfight.
I actually really hope Riot don't buff her. Just because a champion isn't absolutely broken OP doesn't mean she's bad. And again people keep building her like a pure mage and wondering why they accomplish nothing, once people cotton onto the fact she's better as an ap bruiser she'll be seen in a more positive light.