I think it's safe to say that this thing is DEFINITELY the weird baby of Heatran and Tentacruel. The annoying thing about Mollux is that he has some of the WORST four-moveslot syndrome in Pokemon because of his tantalizingly great non-attacking movepool.
Since he can do the jobs of both Tentacruel and Heatran, I always expect him to do both at the same time, but there aren't enough to do both. I find that Mollux works well in conjunction with Tentacruel to defeat Rain teams: one of them serving as an offensive pivot (Mollux) and the other serving as a defensive pivot (Tentacruel). One can set rocks and blast high-powered attacks, while the other sponges hits and Spins.