You learn something new all the time. If I understand this correctly, you can submit a new cache at the location of an existing cache and wait. If/when the existing cache is archived you're submission is first in line to take the location. I nearly stumbled into this while converting my existing traditional to a multi. In order to get the multi approved I'll have to archive the traditional when submitting the multi. With a lurker waiting, like I just explained, he'd be first in line and take my location.
Apparently there's an unspecified limit to the time you can lurk. Typically somewhere around a month. I guess it's at the discretion of the approver.
I can load caches as a GPX file and they show up on the screen as geocaches with the correct icon -- trad, multi, puzzle, etc. Cache description, hints and recent logs are available. The Montana holds up to 15,000 caches (though I seldom put over 8-9,000 in mine). If necessary, I can switch from Automobile driving mode to geocaching mode and use it outside of the vehicle to navigate to the cache. It works pretty much like an Oregon 550 in that regard. You can even mark caches as found, add field notes and upload to GC.com later on, if you want. It comes with a rechargeable battery, but you can take it out and install 3 AA's if you need to.
Doesn't accept GGZ files like the Oregon 650 or the 64s. Firmware can be glitchy. Mine will sometime just shut down when I try to navigate to a cache and it has trouble calculating a route. When I load caches on it, I have to boot it up and make sure they "took" (sometimes the file is in there but the caches just don't show up on the screen). It's bulky and the screen is rather small and low-res for the size of the unit, IMHO.
Kirbydox has the newest Montana 680T, which has 8 gig of memory and a higher mega-pixel camera than my 650T (I never use the camera anyway).