I would also get the basic $600 Montana 700 if I needed a new Garmin today (fortunately I don't). But would be concerned about the size and weight. And I really can't justify the purchase of such an expensive GPS, I just don't need it for what I do today.
They seem to be pretty clear that it includes a "lifetime" Birdseye subscription
The GPS has wifi, so you download it directly to the device. But it would be locked to that specific GPS and wouldn't work on any other GPS. Before the GPS forums shutdown, there was some question as to whether you could download directly to you computer and transfer to the GPS since you don't have a "normal" Birdseye subscription. In the past, you could always download whatever you wanted on your computer in Basecamp but it would need to be authorized by Garmin's servers when you transfer it to the GPS.
I have a Garmin DriveTrack 70 automotive GPS that included a one-year Birdseye subscription. No problems authorizing and transfering imagery to it. I actually didn't download any Garmin imagery but used my own with some software that formats it as .jnx Birdseye files. This fools Garmin's servers and they will authorize it. Processed a lot of it (most of NJ) when I got the GPS and still use it, but the subscription is over now so I can't add anymore unless I renew the subscription. Even when your subscription expires, you can continue using authorized imagery on the original device, but you can't transfer/authorize new imagery without an active subscription.
If you get a new GPS, you should still be able to transfer any existing Birdseye imagery to it from Basecamp *IF* that device has an active Birdseye subscription. However, if you have a memory card that already contains Birdseye files, I don't think that will work at all on the new GPS. You would need to copy the files from the card into Basecamp, then send them to the new GPS, which would trigger the authorization process.
I agree, Garmin doesn't "speak clearly" about Birdseye, but my description above is how it works. Or maybe I should say "how it worked" because I've never had a device with a lifetime subscription. Anyway, I don't have an issue with any of that. My big complaint is that Garmin has gone out of their way to prevent you from loading your own high-resolution imagery. That is why I finally stopped offering my Garmin maps. You can only load a tiny map that you create yourself. This is an artificial restriction to force you to purchase Birdseye instead of using your own aerials. And aside from that, their Birdseye format just sucks in terms of performance. Really slow to zoom and scroll around. There are open source formats - like what I use in my app - that are lightning fast but Garmin doesn't use them simply because they want it locked down to force you to buy Birdseye.