Psychic

Rock Attacks

Water

Rock attacks are most effective against Fire, Ice, Flying and Bug Pokémon.

Rock attacks are weakest against Fighting and Ground Pokémon. There aren't all that many of these guys around, so the weaknesses aren't so bad. However...

Rock attacks suck. They are inaccurate and weak. Many Rock Pokémon can't even learn these moves, making them even less useful, because they can't take advantage of the type benefit. Rock Slide is not a good choice to add to your non-Rock Pokémon as a TM, even if it has a good Attack score, unless you are specifically looking for an attack that is effective against a Charizard, Articuno, Butterfree, or other doubly vulnerable Pokémon.

(To be fair, it should be noted that two of the three legendary birds take four times regular damage from Rock attacks. Additionally Articuno and Zapdos take only regular damage to Ice and Electric attacks, respectively, which are typically strong against Flying Pokémon. So if you've got a legendary bird problem, Rock Slide might be your answer.)

However, Water Pokémon are common, and can take care of Fire Pokémon for you. A good Fire Pokémon can easily take out Ice and Bug Pokémon, while a nice Thunderbolt can deal with most Flying Pokémon you might encounter. In short, you probably don't need any Rock attacks on your team, and even if you wanted one, a more solid attack of another type could probably do at least an equivalent job.

If you really love your Rock Pokémon and don't want to give him up, Rock Slide won't be totally useless, but you'd still probably be better off with a nice Body Slam or Earthquake. Since there are no pure Rock Pokémon, it is likely that you will have a better elemental choice as well.

Another possible usage of Rock attacks is against Ghost Pokémon. Since defense against Rock attacks is based on the Defense statistic and the Gastly series have considerably lower Defense scores than Special scores, Rock attacks are a good way to get past both their immunity to Normal and Fighting Attacks and their high Specials. However, if your Pokémon has access to a good Ground (or Flying) attack, that would be a better choice.

Rock Slide is more damaging and more accurate than Rock Throw...but it still sucks. While it does have 5 fewer PP than Rock Throw, Rock Throw is so totally awful that it's not even a consideration. If you've been having trouble raising a low level Geodude, Rock Throw is probably the reason.

None.