LS is Life Steal. You still level her Q and then level her E. Try to get LS as soon as you can and keep pots on you. If you leave a camp with full fury and full health, you won't lose your fury if you have full health and only lose it if you take more than 10s to get to the lane, which with her movement speed, she can easily make it to the lane in time to press E at the enemy, knock them up, press Q and E at the same, doing a huge amount of burst. This is where she was considered OP. Her E is on a long-ish CD, so it's damage is mostly fine, but because of how her kit works, you can force it to be used at the beginning of the fight with max fury and get off your burst.
Should probably mention her weaknesses since I'm pointing out a trick that'll double a Rek'Sai's burst. She has to E at the target. If you use a knock back when she tunnels, she gets knocked back to the tunnel. This means if she's about to exit the second tunnel and you knock her back, she will be forced back to her other one. It doesn't ALWAYS happen, which shows it might actually be a bug, but it is one of her weaknesses. Another weakness is that her gap closer goes in a straight line and always goes the same distance no matter what. You can use this to your advantage and just... walk AT her to avoid her or just walk away from where she's going to pop up at.
Personally, I find her E's scaling a bit disgusting though, especially since I'm kind of a douche and like to rush a BT, so by the time I have my E and Q maxed, I'll have a BT. BT helps keep you topped off so your W doesn't take your fury. It also gives 80 flat damage which translates to 192 true damage. Her E has a 240% of total AD in TRUE damage. This means if you have about 300 AD, pretty common with a Tri+BT+Warrior Enchant, you'll be doing about 720 true damage. Instantly. They're balancing her normal tankier build though, which is usually a devourer then Hydra, which is a common build path right now. What's bad about that is that with just a Hydra for bonus AD, you're only get 75. I know my own build path is overly expensive for most junglers and most prefer Hydra, so I always recommend Hydra and Warrior enchant. That's 120 bonus AD on top of her 100ish base and 15 AD you likely get from runes plus the 6% of bonus AD extra and of course 18 at max level... You see where this is going. She can have about 250 AD with just a Hydra and a Warrior Enchant. That translates to 600 true damage from a max fury E with only two AD items.
Overall, Riot needs to balance her properly and players need to learn where her burst sits. Her burst is not her Q, it's not her devourer, and it's most certainly not her unburrow. It's that huge burst from her unborrowed E that'll destroy a squishy who only has about 2k health. You're doing nearly half their health in unavoidable damage on a 12s CD, which with masteries and just a warrior enchant is 15% off, which brings it down to about 10.5s. When compared to Cho'Gath's R or Darius's R or even Irelia's W with maxed attack speed, you simply can't compare to Rek'Sai's E. People can say 'Oh, you need fury'. Okay, while your team pushes, take a wraith or wolf camp and make sure you Q at minions when burrowed to stay in combat, thus preventing you from losing fury.