Porygon2 is pretty good. Its history in Adv shows it to be one of the bulkiest pokemon in the game. Now, in BW, it 'would' have been renderered obsolete by the power increase, but Porygon2 has also been given its own weapon in the form of Eviolite, making the duck a very bulky duck.
Furthermore, Porygon2's superior Bulk can combine with its decent Special Attack and extremely powerful Coverage in TriAttack + BoltBeam to damage many common enemies in the game such as Flygon, Slowbro, Gligar, Togekiss, while they can do nothing back to Porygon. Not to mention, Porygon2's spammable STAB attack in Tri Attack also gives it a 20% chance of inflicting major hax(Burn/Para/Freeze, pretty painful any way you look at it) on your opponent.
It's the premier Special Tank in UU(fuck snorlax) and also a very viable Physical Tank.
So Porygon2 is quite a good pokemon.