I haven't ever found it usable since its stats are subpar and its claim to fame is Forecast, which really just wastes an Ability, since Castform is only even decent after a weather condition is set. Basically, i'd just go with a Fire-, Water-, or Ice-Type that fits your weather team already since its stats will probably treat you better, it will have a deeper movepool, and it can have a USEFUL ability instead of a useless gimmicky parlor trick.