I brought the 800mm without the 2x converter, haven't used it much yet, but first shot were ok,issue with focus which was difficult because of stiffness in rotating barrel but that is often better than too loose. Main issue was with lens mount which was a bit slack. But the real issue is hanging all that weight off a camera lens mount. Look for the lenses that have a tripod mount built in. I plan on building some sort of combined lens camera support.
But hey, for $120 I paid for mine it's worth experimenting
