Yeah, Matt, storage is a concern for me too. I wasn't able to swing the trailer purchase and have a garage built as well. Like you, I contemplated storing it at a storage rental facility, but felt the $180.00/month could be put to better use (such as towards the garage I want to have built). I also wasn't thrilled with the inability to plug the trailer in at the storage compound (meaning it would be necessary to remove the battery and keep it trickle charged). Removing the battery isn't a big deal, I just didn't warm up to the idea. I also considered erecting a temporary shelter, but much to my shagrin my City's bylaw office said, "no, no, no". In the end, I decided on going with the storage cover.
My climate is typically dry, so mildew isn't normally a concern. I imagine mildew concerns were likely taken into account though when the storage cover was made. I do recall Cary mentioning that they went to great lengths to find a good breathable cover. I suggest checking with Cary about your mildew question.
I have noticed some slight discoloration, a little bit of cover pattern imprinting around the top area of the hatch, and a few rub marks along the top area of the frame.
Agh, this makes me sad.
. . . But, I am waiting until my garage is built before attempting to buff/polish these areas, so I can't comment on how well these areas can be spruced up.
In terms of sturdiness, I have been using the storage cover for about 4 months and can say that it does stand up very well to windy conditions and heavy downpours. I do wash the trailer after each use (not only to keep it looking as shiny as possible but also as an attempt to avoid scratching the trailer by placing the cover over top of a dirty surface). When I later take the cover off, I don't see any sign of dust/grime having passed through the cover.
You do need to be somewhat careful though when putting the cover on. I did put a small rip in the front of mine. There are roughly 8 reinforced areas on the inner-side of the cover that kind of look like a knee patches, which are strategically placed to protect the cover from protruding parts of the trailer. I was trying to pull the cover down over the hatch area and didn't realize that one of the reinforced cover corners at the front of the trailer shifted out of place (until after I had torn the cover, of course). I sewed the area up and it has held fine ever since. If you have a second set of hands to help you put the cover on, this probably will be a non-issue.
Kelly