Figured I'd chime in on this one. I started the season on an Atomic SX7 (2003-2004 year) and did about 12 ski days on them (primarily at Greek Peak in central New York where I am, but I did get a chance to bring them up to Killington, Sunday River, and Gore Mountain). I hadn't skied in about 4 years, and went from relearning things on greens to comfortably skiing blacks (as long as there weren't massive bumps).
The SX7 is a good ski, but from what I understand, it's not in the same league as the RX6 (think of the SX7 as more of a beginner/intermediate ski, while the RX6 is more of an intermediate/advanced ski). I actually just moved up to skiing a Fischer RX8, and am absolutely loving it (even though, some people recommended I check out the RX6 also). I think the SX9 would probably line up more favorably with the Fischer from what I've heard. I found the SX7 to have a tendency to wash out some in sharp turns, and the amount of flex in the ski made skiing bumps and uneven terrain very difficult. I also found it downright disconcerting if you had your weight come off the ski (whether deliberately jumping or just hitting things at speed). I felt like it was really hard to know how the ski was going to handle when it made full contact with the snow again. The Device 3:11 binding on the SX7 was also a sore point for me, it was a pain to work with and adjust, and transmitted tons of vibration from the ski right into your legs. I'm not sure how the binding on the 2004-2005 model would compare though.
I know the railflex system on the RX6 is not quite as elaborate as that on the RX8 and RX9, but these are great bindings (I preferred it greatly to the Rossi one I demoed while skiing some Z5s). Hope those thoughts help!