Fuck yeah, Pokemon. I own (in order of obtainment) Diamond, Emerald, Platinum, Pearl, Fire Red, SoulSilver, and HeartGold. I'm about 50 pokemon away from a complete nation dex. Argh!
About the is Screech useful/not useful debate above:
Firstly, the exact calculations for stat modifiers is 2/2, adding the number of times the stat has been raised/lowered to the appropriate side. "Stat rose" moves raise the stat once, and "stat sharply rose" raise it twice. So, after using Screech once, which lowers the opponent's defense twice, the opponent's defense stat is at
2/4 the normal, or 50%. Again, and it's at 2/6, or 33%, and if used a final time (the max a stat can be raised or lowered is 6), it becomes 2/8, or 25%. So, the following scenarios are possible:
Attack (100%)
Attack (200%)
Attack (300%)
Attack (400%)
Attack (500%)
Attack (600%)
Attack (700%)
Screech
Attack (200%)
Attack (400%)
Attack (600%)
Attack (800%)
Attack (1000%)
Attack (1200%)
Screech
Screech
Attack (300%)
Attack (600%)
Attack (900%)
Attack (1200%)
Attack (1500%)
Screech
Screech
Screech
Attack (400%)
Attack (800%)
Attack (1200%)
Attack (1600%)
So, it really depends on how many turns you'll have until you die. If the opponent is really bulky, or has some really cheap recovery move, the more screech, the better. If the opponent is fast and frail, it's better to just use screech once, or not at all.
So, for every amount of turns you survive, the correct use of screech changes.
T1: Never
T2: Never or Once
T3: Once
T4: Once or Twice
T5: Twice
T6: Twice or Thrice
T7: Thrice
Pretty basic. If the opponent switches out though, you still get to screech or attack the new opponent, and deal entry hazard damage. However, you might end up wasting a few turns screeching while they attack you, then they switch and it's all wasted. So it's not recommendable to use screech 3 times, and then start attacking, unless you use pursuit, but that requires a good bit of predicting. If you're using Swords Dance or a similar self-effecting move, it becomes easier, though you still have to look out for frailty, and moves like Roar and Whirlwind.
I can't believe I just spent 20 minutes writing, researching, and calculating for Pokemon.