Having played with the new medigun for 4 hours now I'd like to point out a few things.

1. Using this medigun promotes aggressive play, while it does build uber faster both of the other mediguns are better once they have charge. Therefore when you are using this medigun you want to push quickly, or you will be punished for not taking a different medigun.

2. Due to the lowered buff rate you seem to have -150 to -200 health for the midfight compared to a normal medigun. However, you build at least 1 charge (on some maps 2) before the midfight starts if your medic has really good micro and shifts his resistance well you could balance out the health disadvantage by stopping a roamer from hurting your team or saving people from demo damage.

3. The Vaccinator adds a level of depth to the medic, this medigun is much worse than the other options if you aren't skilled. Being able to quickly react and deny damage will be the only way to turn this medigun into an advantage, rather than a disadvantage. Allowing this in competitive gives a new layer of skill in each engagement, and personally rekindled some of my passion for the class.

4. As the previous point touched on, this medigun makes medic FUN. You have to be focused, you have to be quick, you need to know who to prioritize and how to push to take advantage of negating damage types. This made medic something fun, and made it a challenging experience.

5. This weapon is still not OP though, if you are smart you can easily take out a team running this, having your team be extremely aggressive in the mid will give you a large advantage because you can focus down the other players before the medic pops, since everyone on the team running this has much less health than normal.

6. Demos by necessity have to be a half second slower to midfights because their 260 buff comes out slower. Not a huge deal, but still a thing to be noted, also, even with critheals it will take much longer to heal up your pocket.

7. Mixing up your positioning will be very important, and movement/surfing doubly so, this medigun doesn't make you invulnerable, you need to play it smart. Not to say that it's horrible like the Quick Fix, if you pop the Explosive damage reduction even 5 stick traps won't kill you. This also makes team focus fire more important, because the main way to counter this is to get both bullet and explosive damage onto the medic at the same time.

8. The best thing about this medigun is when you have bullet resistance selected you can't be quick-scoped or full charge body-shotted, you'll survive with 5 health.

That's all for right now folks, I hope you enjoy experimenting with this excellent addition to the game in some PUGs, and giving it the opportunity to prove itself.

Helpful tips from others:

Nixnyte
if you do not have a mouse3 or already have something bound to it, i personally found binding +attack3 on top of my slot 2 key to be very comfortable. what i mean is, for me, pressing E switches to the medigun if it's not out, but if it is, it instead changes the resistance type

just something to consider since i've heard some complaints about mouse3 tonight