Few things:
1.) The clunky feel of Kolto Injection can be mitigated by adjusting one of the settings in the game - Pref > Combat > Ability Action Queue Window. Adjust that to your personal preference, i have mine set to .75
2.) Agreed the casting time is annoying, because in addition to GCD and latency, KI also has travel time. Our 2 second heal actually becomes closer to 2.5 before it actually heals somebody, depending on the range of your target.
The travel time has to be the most annoying thing so far, because there have been a few times where the cast timer finishes, but target either los or ranges me before the shot leaves my arm and they do not get healed.
3.) Diagnostic Scan is next to completely useless. I get that it costs no energy, and I can talent it to possibly get 3 whole energy back per cast; but seriously I'm probably better of DPSing while i wait for regen.
4.) Kolto Infusion: only gripe is that the heal should only check if you have Tactical Advantage when it is first cast, instead of at the end. Since our UI sucks right now, while i'm in combat i do not know how many seconds i have left on TA. I can guess, but I see it's running out and i don't know if that's one second or two. I start to cast Infusion, and then as my cast time finishes, TA has dropped of and the heal fails. Sweet, guess I only had one second of TA left.
BUT, I like healing as an Agent a lot. I am hopeful the hiccups will be addressed quickly. Bioware really needs to allow UI customization or allow mods so i can display my UI how i want to see it. Even something as simple as putting the amount of time on a buff, or cooldown window. Healing relies a lot on TA, and not knowing exactly how long you have left on it is going to get extremely annoying end game.