I am a support main and the guides I saw for Rell when she first came out were all in all BAD. I saw people max their E first, Riots item guide was god awful and she is still new.
My first impression of Rell was that she was Leona 2.o a utility support. Then I pressed w and fell in love with it. It is so satisfying watching her jump and land. When she mounts back up it is equally fun as it can reset her aa and deal some good damage. Her q can keep you alive in a pinch and heals a lot of health. her e however is clunky. It has a starting cd of 21 seconds. and it gets better as you level it. q starts at 9 and ends at 4 making it nice and spammable. W the main engaged and disengage. static 10 seconds but sheild and ms increase per a point.

I am starting with cons cause that's what I look at 1st when decide who to main.
She is slow. Really slow after you jump in with w. You will not be able to dodge any skill shots after you engage but that can be fine because your tanky with aftershock.
Her attack speed is adismal. you start with like 0.55 attack speed so once every 2 seconds you do damege.
Her damage is also non existent. If you out-damage people as Rell Report them as they were trolling that game Rell should never ever do more damage then a Yi, ahri, kat, or any non hyper tank.
lastly her E, Rells E feels super buggy. Sometimes it can attach itself back on other times it does not. It has a 3 second cd after you attach it to someone before you can stun or attach it to someone else. It is understandable because it would be busted if you can stun people on the fly but the 3 seconds is defintly to long.

She is Fun, her W feels amazing and rewarding when used right. almost always gets a flash or kill when used right.
She can tank tower for ever making tower dives easy. her passive screws over tanks.

I am a Leona main, I will ruin your game if you steal her from me. You have been warned

If you have any comments or notice anything that I missed do let me know.