The biggest issue I have had is the learning curve. My 999 does so many things that it takes a dedicated effort to really learn the capabilities. You can just turn it on and use it but to really understand what it is showing/telling you you have to dedicate the time. I wasted 6 months casually using it while I concentrated on fishing. For the last 6 months I have actually taken the time to work with the unit and my images are getting consistently better. I now know when to switch frequencies, how to adjust my screen width, and work with the sensitivity and depth settings. It just takes time on the water working with the controls.
O yea, the knowledgeable and helpful people on this site have been invaluable and can save you a lot of time and head scratching.
Good luck