Been wondering about this item and how it works, so I did some research. Came up with a new and better description:
Holder is unaffected by the effects of sun and rain. Holder is still affected by sand and hail.

Here are its various effects:
Damage: Holder's attacks still boosted and nerfed in weather. Opponent's attack boosts and nerfs in weather are ignored.
Weather Ball: Holder's remains normal type. Opponent's has correct type.
Solar Beam/Blade: Holder's always requires a charge. Opponent's still require no charge in sun and still do half-damage in rain.
Morning Sun, Moonlight, Synthesis: Holder's always restore 50%, unaffected by weather buffs/nerfs.
Growth: Holder's boosts are not doubled in sun.
Chlorophyll, Dry Skin, Flower Gift, Forecast, Hydration, Leaf Guard, Rain Dish, Solar Power, Swift Swim: Holder's are prevented from activating. Opponent's Swift Swim, Chlorophyll unaffected.
Hurricane, Thunder: Holder's still %100 accurate in proper weather. Opponent's moves can still miss.

What to take away:
Although it carries some negative effects, it can't really function well as a Trick item. Sure, it'll force Solar Beam to charge, kill Weather Ball as coverage, and negate a handful of weather abilities from activating. But this just makes it incredibly circumstantial and won't even reduce the damage they're doing under weather.
This means you'll probably want to use it as a beneficial item most of the time. It protects your mon from taking boosted Surf or Flamethrower, while still allowing your moves to be boosted. It also turns Synthesis and co. into reliable healing regardless of weather conditions. It also protects against 100% accurate Thunders and Hurricanes from your opponent, harnessing RNG to your benefit.