I bought my humminbird 987 Si about 4 years ago at what I considered a reasonable sum for Side imaging capability. The unit has been awesome. Have found a few wrecks, identifying the shape of rock formations, bottom hardness and scouring is all possible. All this talk of Down Imaging from Lowrance got me quite interested in the Lowrance units.
Last week Humminbird released a software update for their units to introduce Down Imaging, my unit at the time was not included but has since been added. Great service, a 4 year old unit with next generation software for no upgrade costs. Will be testing the down imaging this week so will save some screen shots (this is so easy as they get saved straight onto an sd card).
The only things that I would like to change for are a bigger screen for the chartplotter and more power / range when using sidescan. The side scan is good to about 40m depth. The usual 2d sounder is perfect in much deeper water and you can track your fishing rig 50m below the boat with ease.
The humminbird forums are a fantastic resource and the free PC software is great for managing wpts and routes. The 3rd party software available as mentioned above looks really exciting but I have not had time to try it. Would like to give Yellowfin a go sometime soon.