Yes, you must sign out of PSN, or the game will automatically start in online mode.
I think the only thing you miss out on playing as a hollow is the ability to summon not only players, but NPCs that youve met along the way. I dont think youll miss anything else, but I may be wrong.
Hopefully someone else can clarify or correct me there.
Whenever I want to summon an NPC, I sign out, go human, kill the Boss and then go back online.
On the rare occasion that I co-op, theres always two or more of us, so taking down invaders is usually pretty quick.
No voice and no text. Its all hit and miss, or you could hash out a basic plan with PSN messages before summoning.
The PvP, Co-Op and online arent very good, HUGE lag spikes and more failed summons then successful ones.
Demons Souls PvP/Co-Op is much better.
Summoning and being invaded is a very important and very fun part of the game, however, since people have discovered the glitch that transfers items from high level characters to low level ones, it's made invasions early-on unbearable.
Trust me, when you're running around the Undead Parish with your starting gear and get invaded by a guy with a Lightning Zweihander +5, it is NOT fun.