Sejuani gameplay
I'm not sure about her balance but I find her gameplay really fun now and this is just from Practice Tool. With old Sejuani I often played her with a full out health build and rode around hitting enemies with W just for the feel of beating up everyone with the flail. Now the cavalry gameplay feels even more real especially with how the new deadman's plate works. Running around aiming W in where you expect the enemy to be, you'll often have around ~440 MS along with landing W with Q. Also you get the cool E->R combo. Chasing down the bot with W, running around them, getting E and R. Feels good man.
I have not played any real games with her so I have not seen her gameplay with melee champions. I feel her synergy with melee champions will be really high though.
Once again I'm not sure about her balance atm but at the end of the day those things can be changed with numbers alone. Gameplay for me gets thumbs up.