Just got the 688 for Christmas, still new in unopened box. There are still 598ci HD SI's available online & I'm thinking about returning the 688 & buying a 598. The 598 has better resolution 640 x 640 vs 480 x 640 for the 688, but my concern about the 598 is around the Sonar Frequency The 2D is 83/200 kHz, while on the 688 it's 200/455 kHz 2D. I know the Side Imaging is a great added feature on the 598, but I'd probably run the unit in 2D as much as DI and or SI. I'd also consider the 698ci HD SI, but it's more than I'd like to spend & the resolution is the same as the 688.
I'd appreciate your thoughts and input.