Had my first real experience with the multiplayer last night. Prior to this the only thing I had done was get invaded while I was stupidly sitting in the beginning of 2-2 in body form screwing around with my inventory and got backstabbed.
I had done multiple runs through 1-3 and 4-1 over the weekend without beating either boss, and since I knew both levels pretty well I decided to go help someone else out. In 1-4, I went Blue Phantom, and beat the boss with the dude. Revived, and summoned 2 Blue Phantoms. Halfway through got invaded. I stabbed him with my baby's nail, plauged him, and his health started dropping. He charged me and my Blue Phantom started wailing on him. I panicked cause I was on the narrow stairs leading up to where the 3 archers are across and just backed up with my shield up. Just as his health dropped to zero he got one last hit in after my guard broke and we both died at the same time. Went through as a blue phantom again, revived, summoned and got invaded again, kicked the Black Phantom's ass and beat the level. Had so much fun that I suicided in the nexus so I could run through 4-1 as a Blue Phantom before I summoned. Went pretty similarly, except that I didn't get invaded at all as the host on 4-1.
People put too much emphasis on the negatives of the online aspect of the game. Sure, when you are in body form, it's scary as hell to get invaded, but it really stacks the odds in your favor if you have a Blue Phantom or 2 rolling with you. Honestly I think that it probably wouldn't be as fun if you didn't have the possibility of getting invaded while you were running through with Blue Phantoms. It probably would almost feel too easy. I had a blast playing through those 2 levels last night, and can't wait to play more. I can see why people get addicted to this game.