I just had a great experience in Painted World of Ariamis.
I started to go through by myself but since I figured it was a side quest of sorts (not sure if this is true or not - nothing in there seemed "main") I might as well spend a humanity or two and summon players.
The first bloke I teamed up with didn't get too far with me because another player invaded and we both got swiftly killed. It was pretty cheap because the invader lagged so much he backstabbed me even though the game said he was miles in front of me.
The second guy had much more luck with me and we made it all the way to the dragon. I didn't really have anything to combat it other than arrows, but thankfully he was a pyromancer and had an insane fire throw that did 700(!) damage to the dragon. It went down quickly and then we were invaded. I instantly thought we were going to get raped again but we both turned around on the bridge and created a defensive line. It wasn't the most graceful of fights, but we managed to take turns slashing/using magic against the invader and he went down. Boom! Unfortunately, he then died on the Phalanx so I went back to the bonfire to level up and summon again.
This time, I got the same guy as the first time and we went to kill the Phalanxs (thankfully he had good Pyromancy and made quick work of them) and wandered around the sewers etc. Made it to boss, killed her, and voila.
It's amazing how much safer I felt just having another person around. I still nearly got killed a few times, but it was great to be able to roll back and have someone else pick up the slack. It makes me wish the game had a true co-op mode so my girlfriend and I could play with each other. PVP definitely isn't for me, but after ignoring co-op in all of Demon's and most of this game, I can definitely say I made a mistake. I'm going to farm some humanities to get some help in later sections and go back to earlier sections to allow people to summon me, I think.
Also, I got the Key of Annex in the basement but I didn't look around to see if I could use it in the Painted World of Ariamis. There were also a few souls lying around I didn't get around to picking up. I ran back to the bonfire and saved the game for the night, so I hope I can go back to explore it more fully. I'm sure I can. *crosses fingers*