Sadly, I never really used it much, but I definitely have used it! On my first few run-throughs of Hoenn, I would keep it on my team, but after it began to be one shot by Pokémon of an equal level, I decided I needed something a bit more resilient, or that could at least dish the damage out. For this reason, around Fortree, I always picked up a Manectric if I hadn't already trained and evolved an Electrike. In OR/AS my team was full by the time I reached Norman, so I already knew exactly what team I wanted, and Castform just didn't fit, though had there been room, I would have used it again, if only for the nostalgia!
It's a shame so many people think it's useless, since I think with a little bit of perseverance that Castform could potentially be a great in-game Pokémon, perhaps if received a little earlier?