Metronome
Increases the power of moves used consecutively by 20% each time it is used, up to a maximum of 5 turns (100%). If the holder switches out or uses another move, the boosts are lost. Metronome will not power up individual hits of multi-hit moves (such as Rock Blast) unless they are used consecutively.
Competitive use
Metronome isn't a good item. In competitive battling, Pokemon switch frequently, so gaining boosts will be hard. In addition, using one move repeatedly will put you at a disadvantage. Not to mention that even if you use the same move for four turns in a row, you only obtain the same power as a Choice Band or Choice Specs—in fact, even if you used the same move six turns in a row, you would have done exactly the same amount of damage had you had the Choice item to begin with.
Micle Berry
If the holder's HP reaches or falls below 25%, then the next move has its accuracy increased by 20%. This Berry has no effect on Guillotine, Sheer Cold, Fissure, or Horn Drill. Consumed after use.
Competitive use
If this Berry happened to work with OHKO moves, it would be great. Well, since OHKO moves are banned regardless, it wouldn't be that great, but it would be pretty good in any environment where they were allowed, such as VGC or Battle Subway. As it stands, though, there are very few moves that are so powerful regardless of accuracy that they would merit the use of the item slot for Micle Berry—even sleep moves like GrassWhistle and Hypnosis are pointless when you could just use a Pokemon with a better sleep-inducing move if you were that desperate.
Wise Glasses
Increases the power of all of the holder's damage-dealing special attacks by 10%.
Competitive use
Wise Glasses are the special equivalent of Muscle Band, and can be pretty useful as an alternative to Life Orb which does not cause recoil. Even though it is a lot weaker than Life Orb—and yes, the difference in damage is significant—it boosts the damage of all your attacks, unlike Expert Belt, which occasionally will not boost them. Expert Belt is typically the favored item, as most Pokemon who want to be switching attacks will have enough coverage to make better use of Expert Belt, and most Pokemon who are relying on STAB attacks will want Life Orb or Choice Specs for the maximum possible power. Wise Glasses is typically used less than Expert Belt both for attacking and for bluffing a Choice item, but the potential is there, if you want to risk it.
Muscle Band
Increases the power of all of the holder's damage-dealing physical attacks by 10%.
Competitive use
Muscle Band is the physical equivalent of Wise Glasses, and can be pretty useful as an alternative to Life Orb which does not cause recoil. Even though it is a lot weaker than Life Orb—and yes, the difference in damage is significant—it boosts the damage of all your attacks, unlike Expert Belt, which occasionally will not boost them. Expert Belt is typically the favored item, as most Pokemon who want to be switching attacks will have enough coverage to make better use of Expert Belt, and most Pokemon who are relying on STAB attacks will want Life Orb or Choice Specs for the maximum possible power. Muscle Band is typically used less than Expert Belt both for attacking and for bluffing a Choice item, but the potential is there, if you want to risk it.
Rowap Berry
If the holder takes damage from a special attack, then it will consume the Rowap Berry, and the attacker will take damage equal to 12.5% of its maximum health. Natural Gift becomes an 80 Base Power Dark-type attack when the user holds this Berry, but the Berry is consumed after use.
Competitive use
Interestingly enough, Rocky Helmet deals more damage than this Berry to the attacker, without even being used up—albeit, to users of contact moves, which are usually distinct from special moves. But still, this Berry deals very little damage to the opponent for the huge cost of losing your precious item slot. If it were dealing 50% damage for any attack, then it might see some use. But at present, it's pretty much unusable.
Starf Berry
If the holder's HP hits or falls below 25%, this Berry will raise one of the holder's stats at random by two stages. Consumed after use. Natural Gift becomes an 80 Base Power Psychic-type attack when the user holds this Berry, but the Berry is consumed after use.
Competitive use
Definitely the best-looking Berry, bar none. However, it's not the best in competitive battles, unfortunately. A free two-stage boost is definitely nice, if of course it is in the stat you wanted it to be in. Speed, evasion, or your preferred attacking stat are always nice—but then it always depends on the situation, meaning that on average, you are unlikely to get the boosts you need even half of the time. Alas, the rainbow Berry simply pales in comparison to more conventional boosting items.
Thanks to smogon. I hope my answer helped you. Stay awesome DB users.
