I've used Firefox for a long time, but I've found Opera to be a very nice (and fast) browser. Unfortunately, there is no Xmarks extension available for Opera. Do you have any plans to provide an Opera extension?
It would be handy to be able to bookmark stuff in Opera and have it available on Xmarks. The only way I've found to get Opera bookmarks into Xmarks is to copy the URL from Opera, paste it into FF/Chrome/IE and then sync. I'll admit I occasionally use LastPass to save bookmarks (Tools > Save All Entered Data) if I'm in a hurry, but it's an awkward way to do it.
I remember there was some issue with Opera's bookmark format being proprietary some years back. If that's still an obstacle, just provide an Xmarks toolbar or toolbar button that will open up a separate set of bookmarks. Sure would beat copy'n'paste...