Pros: YupI'm a frequent user of P2, so I can fill in a bit about Porygon2.
Decent Mixed Wall
Cripples: Salamance, Gyarados, Heatran, Jolteon, Vaporeon, Flygon (come in on a EQ, trace Levitate, Flygon's almost always choiced)
Can carry Thunder Wave, and has the defences to use it.
Switch-ins are met with Thunder Waves
Fighting Weak, thus invites Lucario to kill/set up, and Scizor to... kill/set up, or Infernape, to kill/set up.
Little use at stopping anything except those listed above, although, not much likes a Thunder Wave to the face.
Despite decent sp.attack, lacks power.
Dislikes being poisioned and especially dislikes Toxic Spikes. (Although, bringing in on a Breloom is fun when TS is up ^_^, as you absorb the sleep, then Ice Beam it to death, and get recovery AND status immunity.)
Cons: About the fighting weak: Gyarados is a nice partner together with a rapid spinner and cleric :P that's why P2 fits in most Stall teams, and some balanced teams.