Okay, a bit more information for you Don.
I wear quite dark polarized sunglasses and so far on what have been beautiful riding days, not a cloud in the sky type days, I've not had any issues seeing my screen. All different screens show up very nicely.... Except for the EQ screen. All of the bars and setpoints disappear from view.
I've not used Apple Carplay much, in my new Ford F-350 some just to get a bit of a hang of it, but I'm not enamored with the less than user friendly means to get around on the MAPS screen etc., but that's probably normal for lots of GPS type head units. You cannot just touch the screen and move your location around or zoom in etc. You have to use the 4-points compass icon and then that gives you an arrow on each screen side/top/bottom etc. to push with your finger. To zoom in/out, you use the + - screen buttons. I've been spoiled by how easy it is to use the maps on my iPhone. I'll get used to it for sure. When using Google Maps, the course lights up in blue as normal and the heading seems to be N/S on the screen always. So, your directional arrow is heading in the direction relative to N/S. The little lady speaks very loudly and is quite easy to hear while riding at speed. Cuts the music off and then talks.
I opted for a Bluetooth device to link my iPhone with the Soundstream rather than the hardwire USB as shipped. The one I bought has some lag in connecting, but once connected, it seems to have very little latency for screen manipulation etc.
Neal is correct, no inputs to the Soundstream other than the MIC for phone use. I should have put the MIC on, but didn't. I could have just tucked it in somewhere and I would have it if needed later. But now it would require ANOTHER front fairing removal, and I've done that enough times lately to not want to do it again anytime soon...

Once logged into your phone, you only get about 10 or so of the apps off your phone. I'm guessing that they are the embedded apps that are Apple only etc. When running MAPS, you can split screen to see what XM is playing along with half the screen for the GPS. That's nice.
I'm very happy with the new head unit. I would highly recommend it for people that want a bit more music options and to have GPS on the bike and not on your phone on the handlebars. For some, it's a grand waisted. The ONLY reason I could have had to upgrade my '09 SERG, was the Rushmore upgrades and the new Harley screen head units. Now for a grand, I have what I want and am very happy with bike's performance and comfort. For an '09, still under 25k miles...

I've attached a picture of the inputs for the Soundstream. Looks like if you have aftermarket amps and have four speakers, RCA connections would get one up and running very quickly.
Don't know how the Garmin is going to play out for your guys. I almost did that for mine, but then we stopped riding as much and it didn't seem worthwhile for the money at the time.
If you're worried about the brightness of the screen in bright sunlight, I've not had any issues so far, except for the EQ screen as mentioned above. Not to sound too redundant, I do really like this so far and it looks amazing, just as Harley should have done!
Dan