I just upgraded from a 999 to the Helix 12si. They both use the same Hidef transducer but that is where the similarities end. I have only been on the water three times with the Helix but I really like it. The picture is so much better: larger, brighter and clearer because it is a much higher resolution screen. I suppose it is the faster processor but my SI/DI images are cleaner at slightly faster speed than the 999. I haven't even started learning about the Chirp sonar or the Automap functions yet. I am sure I will have questions when I get to these.
I can say that the Helix operates exactly like my 999 so everything I learned on the 999 I carry forward. I will master the additional features of the Helix as i need them.