I've used this for a good while now to merge the bookmarks for IE, Firefox, Opera. You need to go into Settings to make sure the paths to the various bookmark files are correct. (Check the path to your opera6.adr file in particular.) Once you have it set up, you just click on Merge and it will synchronise all your bookmarks. (It doesn't synchronise in real time, only when you click on Merge.)
(For example, you can use Firefox for a time, then Opera and IE afterwards. When you use BookmarkBridge to merge your bookmarks it will update any additions or deletions you have made using all three browsers. Firefox, IE, and Opera will have identical bookmarks.)
Although I've never had a problem with it, before using it for the first time back up your individual browsers' bookmarks.
About the opera6.adr file. You can find the location of this by going into Help in Opera and clicking on About Opera. It will be in one of two locations depending on whether you installed Opera for one user or multiple users.