My dive buddy got one for their 20'ish Apex. They were looking forward to being able to ID new dive sites, and see exactly what wrecks looked like and such.
Their current evaluation is that it doesn't work. Last weekend, they tried getting a shot of a 40' sailboat hull that sits in about 25' of water. Carol said she saw a blob, and that was it.
Here's what she posted on the exercise:
While waiting for Jeff and his buddies we cruised around running my Humminbird sidescan over the barge and the sail boat to see what they looked like on it. The barge was pretty much unseeable.
The sailboat showed up well, but didn't look very boatlike. All in all, I wouldn't buy the Humminbird again. That feature is pretty useless, but not cheap. :-)
Note: The Barge was a 60 to 80' sailing vessel that had been dismasted and used to tow cargo around the Monterey area. It apparently burned and sank (and not necessarily accidentally.) It's now just a series of ribs and some planking sitting in 65 feet, with a relief of maybe 3 feet at the high points.
The sailboat, as I said, sits in 25 feet, partially buried, and has a relief of at least ten feet. It *may* be made of cement, though I suspect it's glas. I haven't been diving on that particular site for many years, so the current data may be a little suspect. FWIW, my Lowrance apparently picked up about the same echoes off the sailboat as her Humminbird thingie.