I have just finished fitting a new Lowrance Elite-7 HDI (combo chartplotter and Sonar) in my boat.
I had two units before, a garmin Montana 600 chartplotter and a Garmin 400C sonar. Note - I don't call them fishfinders. The 400C is a very cheap (about $450) colour unit with 4" screen. It shows the bottom well and structure - it showed schools of fish but didn't seem to show any individual fish. I wanted to upgrade the sonar to show fish like I saw on pictures of units like Lowrance HDS.
I bought a Elite-7 HDI with Navionics Gold charts NZ included for $1250 - a good price I thought. Went through the usual palaver of fitting the transducer, running cable, and connecting the unit to the 12v power. It appeared to work ok - BUT (there's always a but).

One thing I wanted was a built-in compass. Now, I know marine chartplotters don't include compasses, for some unfathomable reason - they simply try to work out the boat heading, by calculating the direction between two recent gps positions. And often get it very wrong (particularly at low speed). I have had a compass in the Garmin Montana (hand-held) gps, and love it - I wouldn't go back to the old inaccurate heading. So, I bought a separate device, a Lowrance Point-1, that is a gps external antenna (not needed), and an integral electronic compass. It connects via a NMEA 2000 network. I connected it to the NMEA 2000 plug on the Elite-7, but it didn't work - the Elite didn't appear to have anything to recognize the Point-1 for heading (even though specs said the Point-1 would work with the Elite-7).
Research on the internet found a software update, that included support for external heading from the Point-1. So after some mucking around (had to get help from Navico NZ to advise how to download the software update onto a SD card, then input it into the Elite-7). By the way - the software update deleted all my waypoints that I had entered laboriously!!!).

Ok, the Elite now showed a facility to get external Heading, but didn't recognize the Point-1). Back to Navico NZ (very helpful by the way) - I hadn't realized that the NMEA 2000 had to be made up of more connectors and cables and powered from the 12v power.

I decided to step back and review my options. Another factor - the Elite-7 isn't touch screen, whereas the Garmin chartplotter is. I find the Garmin Montana extremely good, very easy and quick to operate (doing chartplotter things like tracks and waypoints) - but the Elite-7 difficult and slow to operate. I was already missing the Montana.
So, I decided to use the Elite-7 for sonar only - giving a bigger 7" screen devoted wholely to sonar. And keep the Garmin Montana for chartplotter - a 4" screen, about the same as half the Elite-7 screen.

I went out yesterday, and it appears the Elite-7 sonar is a big improvement over the Garmin 400C. Just beyond launching out from Maketu, in 6m, I saw a big school of fish on the sonar, plus bigger fish arches around it. Aha. I dropped a bait and caught a Kahawai. Later I examined clear structure and fish signs, near Motiti island.

Incidentally, the Elite-7 has standard sonar (said the same as the HDS units), plus a second sonar called Downscan, that appears to show the same picture in a different colour - no advantage that I can see.

So far, I'm pleased with the Elite-7 sonar, and glad I didn't persevere with the Point-1, that seemed a complicated over-kill on my little 4.1m boat. I will sell the old garmin 400C, the near-new Point-1 and the Navionics Gold NZ SD card.

I should have found out more before - you live and learn.

Oh - one more thing about the Elite-7. I find the transducer mount pathetic - very complicated and far too flimsy, doesn't hold the transducer firmly enough to stop it flopping around.

Anyone with ideas on the Elite-7, I would love to hear.