I didn't "change" characters. I made two builds that weren't possible before I19. The extra pool powers allowed me to get both a Spines/Fire Scrapper and Dark/Fire tanker to 32.5% defense on melee, ranged, and AoE. I love being almost unkillable. Prior to I19, the characters were collecting dust without Stamina to make their endurance manageable.
Only 45 on my kat/wp scrapper currently, but went from slightly challenging on 0/x2 (no sets yet) to slightly boring on 0/x3. Woot!
Also went from an unsustainable attack chain to a completely sustainable attack chain even with 8 toggles running.
So yeah, pretty darn nice in my book. 
Adding Leadership on all my characters. I like it. Can't say I've noticed a huge difference, but knowing it's there is enough - and as most of my playtime is spent on teams these days, it helps the whole team, not just me.
Most of my scrappers are WP who never took fitness anyways so there wasnt much to swap out. For the SR scrappers this allows me to get the passives alot earlier for exemplaring. I am more so interested in how the extra power(s) can be used in conjuction with the alpha slot to make building for the soft cap or higher regeneration numbers easier.
Bump and Grind Bane/SoA
Kenja No Ishi Earth/Empathy Controller
Legendary Sannin Ninja/Pain Mastermind
Entoxicated Ninja/PSN Mastermind
Ninja Ryukenden Kat/WP Scrapper
Hellish Thoughts Fire/PSI Dominator
Thank You Devs for Merits!!!!
For Kitten America (claw/SR/body, level 50) I was able to take tough and weave substantially earlier in the build (yay, better exemplar performance), squeeze the medicine pool back in, and take Elude (not that she needs it). I haven't really tested yet for results.
The Smokemeister (dark/dark/dark, level 47) got a total revamp. He's definitely quite a bit stronger, now. Have some end issues, but once I get 50 and get my alpha slot, he'll be going into the cardiac tree and that will be no worry anymore. My suspicion is that he's going to be my strongest overall scrapper, at least until I level up my katana/WP.
Impulse Drive (KM/WP, level 37) didn't have fitness before, so now she just has more regen and more recovery. Bonus! Already strong character gets even better.
Bladewhisper (katana/regen/body, level 50) got a substantial revamp as well. I still don't have all the IO's I need for her and am currently broke, so I haven't put the new build to the test yet, but it should be stronger. I am seriously considering taking her villain for a while and going the Soul Mastery route with Shadowmeld, however. Once her Alpha slot is unlocked, the recharge and healing boosts should be... very nice indeed.
Skylancer (energy/energy/force blaster, level 50) got explosive blast when she didn't have it before. Otherwise, no big difference.
Red Diamond (energy melee/invlun brute, level 50) is being rerolled as super strength because I hate hate hate hate hate EM.
When I19 hit I had planned out my build and went over and over on my build until i found one I liked.
in the process on my main fire/sd I added +5% global recharge, went from 14hP to over 20hp per tick added 10.5% damage. Added more HP added more endurance though i upped my endurance burn so no real gain
What I Noticed is this had a great net + for my scrapper and i have to use aidself a lot less in my big battles
has anyone else noticed a big + to your build with this change
Pinnacle
Langar Thurs-Katana/SR 50; Hejtmane-DM/DA 50
Rogue Spear-Spines/DA 50; Hypnosis-Ill/Rad 50
Sir Thomas Theroux-DM/SR 50; Melted Copper-Fire/Shield 50
Byzantine Warrior-DB/ELA 50;Blade Tempo-50 DB/EA