My group has just gotten Suishen, they were lucky enough to keep Helgarvel around to fetch it, and they gave it to the two weapon fighter. Needless to say, they destroyed the rest of the Ravenscraeg. The party is an all fighter party, so as a group they do tremendous damage to begin with, but I was expecting Kimandatsu to give them a decent challenge. Instead, Suishen did what it was built to do, and carried them through the whole fight without any of them ever taking damage. I thought Kimandatsu had earned herself a break when she managed to charm monster the two weapon fighter, but the player suggested that Suishen just use its ego to dominate its wielder to force him to continue to fight. It did just that and Kimandatsu could do nothing to stop its onslaught.
My big concern with Suishen is that it will make all the rest of the fights too easy, even for a group as poorly balanced as mine. It has eliminated almost all weaknesses the two weapon fighter has. Because its caster level is 20, all of its buffs are effectively infinite, and even if I use charm or fear effects, Suishen can just use its ego to dominate the player (he needs to roll a natural 20 to prevent it), which practically gives him immunity to such effects.
To me it seems like a mistake to give the characters a minor artifact in part 2. Has anyone else had problems with Suishen, or any other thoughts about it?