I tried the Rescue Ranger and Vaccinator a bit last night, I didn't have any spare L&Ls to craft a Loose Cannon.
Regarding the Rescue Ranger (bonus points for alliteration :V), opinions on the server I was on varied a bit. Personally, I thought it was very useful, but not overpowered. The shotgun has a 4/16 clip, which is pretty small. The main knock on it was that the shots can't remove sappers, which is the main threat to an Engineer's buildings. The ability to remote pickup and repair at range were both good for saving buildings on the move. The self mark of death on pickup wasn't much of a hindrance.
For the Vaccinator, most people liked it. The 10% damage resist on selected damage type while healing is definitely the best part, but the Uber effect pales compared to the regular Ubercharge or Kritz. I had to mind the duration of each section of charge carefully, as having 4 pieces played a bit different than having 1 longer charge.