Gunboats pocket is very powerful, especially in the current, highly aggressive meta. It does, however, cause notably weaker ubers.
If someone else hasn't echoed my thoughts, I'll go a little more in depth when I get home by editing this post.
edit:
Most of the points have been hit and I'm lazy so this'll be super brief. If you think about how soldier can take damage, there's (should) really only be two sources, the enemy and a rocket jump. Gunboats lesser the amount of damage from rocket jumps. This has two major effects, soldiers can move from place to place faster by using more rocket jumps for the same amount of health and soldiers have more opportunity to rebomb and reposition (counter jump). With gunboats, soldier is significantly more heal efficient in providing both space for his team and damage in overly long team fights. The heals that would have been given to a shotgun soldier can instead be given to a demo or scout, who can in turn play significantly more aggressive and as a result the whole team tends to be more aggressive when pushing. With gunboats, if played properly, there is more space, more available heals and more aggression.
This season in invite, I would venture a guess and say at least half will use gunboats regularly on certain maps, because it is map dependent. Even heal heavy players, like goldfish, take significantly less heals when running gunboats and can be surprisingly more aggressive. It is map dependent, for maps like sunshine, viaduct and process, gunboats are especially effective because of how much space there is to bomb in general and how open the chokes are to jump through. For maps like granary and metalworks, and to a much lesser extent badlands, shotgun is stronger for most of the map simply because of how the chokes slow down the game and how separated the flank and the combo will become from each other. As for shotgun pocket, it provides a lot more opportunities to take 1v1's and be successful and lends to much stronger solo ubers if not all ubers.
As for the demo nerf, demo is still capable of an extremely high damage output, especially if they're given the space to walk into the other team shooting stickies. If anything I would say the nerf actually helped this playstyle because scouts can chew through things just as quick and without as much threat from a demoman who is playing safe because longer range stickies are harder to hit (which I wouldn't say is a bad thing :D).
@i52 only lansky really ran it
mike, zebbo and yuki all ran shotgun significantly more than gunboats, if they ran gunboats at all