I've just read the first two chapters, and I'm most impressed. It's been a while since I've read a Pokemon fic that felt this original, which is odd because many of the individual aspects of the story are so conventional. The stranger inhabiting the main character's body is nothing new, the whole deal with Magma/Aqua and the Devon Goods are straight out of the game, and the idea of a human having a Pokemon's power has also been done. Make the stranger a Rotom and put it all together, though, and you have gold.
The first thing that really caught my attention was the way the narrative was split apart at the beginning of the first chapter, and the threads gradually got closer together until Puck actually enters Kester's body and the threads converged. Very clever use of structure. It feels like a collision both because of what's actually happening and how you lay it out.
Character-wise, I'm seeing some really good signs. Kester and Puck have good chemistry, and most importantly their interactions are funny. I also like Kester's mom, even if the whole checking to see if the kid is alright and then grounding him routine is a little cliched. The best touch in terms of character, though, is undoubtedly Puck's English nationality. People often forget that the Pokemon world's geography and history at large is mostly like ours, and it makes more opportunity for humor.
If there's one thing that I thought was a little out of place, though, it was the name of the "P-L.O.T. Device." Judging by its name, I'm going to guess that it has some importance later, but when I first read it I thought it seemed closer to straight-up parody, which I didn't think you were going for. Not a big issue, and probably something you can't change now, but I felt I had to say it.
It seems I have a lot of catching up to do, and I'm looking forward to it!