The ability to sync between Safari, Firefox, and IE bookmarks across multiple machines is very handy. Especially if you have to view certain content within different browsers and you want to access your bookmarks easily without creating more than one bookmark archive. Also, it is much easier for backup purposes to consolidate all your bookmarks to one archive.
There have been many attempts using Terminal commands and Automator to try to make this work, but the most easy, efficient and free way to do this is Foxmarks. It's a great little app you can install on each machine you want to sync. It has a nice feature which lets you edit your bookmarks on the fly from the web on a remote machine. You can view any of your bookmarks immediately from a cellphone or handheld device with a web browser. You can also decide which bookmarks you want to sync with certain computers if you do not wish to sync everything. The best part is that it is free.
Currently, there are downloads available for Firefox 2 and 3, IE 6 and 7, and Safari 3.