I use both Chrome and Firefox, and I also use two user accounts in Windows. It's really important that I keep all my bookmarks synced between all 4 browser instances. 

Up until now I've been using XMarks but I've come to terms with the fact that it's a useless heap of unmentionable. It just doesn't work properly, it has often failed to sync new folders I've created and tonight during a mammoth bookmark sorting session it managed to lose all of my bookmarks entirely (luckily I'd backed them up before I started). 

Really looking to use a new service now. Anyone have good experiences with another one?