My personal anecdotal experience from using it on Ubuntu is that Iced Tea isn't as reliable or compatible with Java code as the Sun java. Maybe that's changed recently?
For anyone who's curious, OpenJDK was (if I recall correctly) originally a fork of the early JDK7 code line, but with everything JDK7-specific stripped out. Originally it was fairly buggy, so the initial Iced Tea builds of Open JDK6 weren't really usable for production work, but since Open JDK6 was released most of the bug fixes that have gone into the official JDK6 have also been ported to OpenJDK6, so the current version is now fairly stable and should be comparable in quality to the official JDK.