That they sell....
Personally, I can't say that I've ever seen a Standard Horizon GPS on anyone's boat that I've been on. But I've seen plenty of Garmin and Raymarine on the lake.
I have Garmin, that I purchased on my own.
And Raymarine, because it was already built into the boat that I purchased.
I like the Garmin as it can accept the Bizer chip. Raymarine has their own updates, but I'm not sure if it's as accurate as the Bizer. I haven't compared them lately.
Note: If anyone else has the Navionics charts, you may want to update it again (if you have an old chart chip, you can get an update by purchasing Updates+, about $90 for a year worth of updates). I found some errors on the lake last year, and this past winter, Navionics told me they fixed the error (basically, some rocks were moving from one position to another by choosing 'Nav' vs 'Fish' options on the display), so they said their new charts update fixes the issue. As I'm not on my boat yet (it's still covered in plastic at this point), I haven't verified the fix yet.
By the way, you can check out the Navionics chart of the lake by poking around here:
http://webapp.navionics.com/?lang=en