That was true when it first came out; the version shipped with Ubuntu was the 1.6.0_0 version of Iced Tea, which had all sorts of problems. The latest version is fine.
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.
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.