The hardest part of PvP with 5-box locks is dealing with AoE fears from the enemy, whether in BG's or in Arena. On the otherhand it is utterly brutal the havoc you can wreck though with 5x seeds!

As for running instances, felguards do a pretty decent job of maintaining threat, but they don't have enough health to take blows for long. It'll be interesting to test non-heroic instances once the 3.0 patch comes out, with the Demonic Empathy talent (healing your pet for 15% damage you do).