I did obtain this Castform during my Pokemon Ruby playthrough, male with Careful nature. He's currently in his 40s level-wise and sitting in the PC. I did take him out for a while and played around with his weather strategy and all, plus it's so cute!
And I know a certain someone who adores this thing... (yes, it's Michonne lol)
Castform is not that great when it comes to battling, as its stats are quite low. I really like its concept and design, though. This Pokémon's got such a cute-looking face, and its size and shape make it very easy to hug. n__n And it's pretty creative how it changes its form depending on the weather. My favorite form would definitely have to be sun. After getting a hold of Castform, I didn't do much things with it, unfortunately. I've leveled it up by two - three stages by battling against a few NPCs, and put it into my PC box after that. Like I said, Castform is just not a good Pokémon to battle with. If its stats were a bit higher or perhaps if it had a wider range of attacks in its movepool, then maybe it would have been.
On my first playthrough of Emerald, I got Castform, dropped it off in the PC, and that was the end of it. On my second go-around, though, I decided to use it on my team. It actually was a pretty solid team member! I remember using its weather-manipulating moves to my advantage in gym battles and with the Elite Four. I think it ended up at level 72 by the end of the game. That same Castform was transferred to Platinum, then Black/White, and to X/Y next!