I gave it a try (via Google Reader of course), and was not too impressed for one simple reason. I have to tell it to "go offline." So I guess the idea is that I know I'll be offline later, so I will "go offline" now, thus downloading content to read later. Ok, that's fine, but not very useful. It requires too much planning, in my opinion.
It's really not that big of a deal for Google Reader. But clearly Gears has the most potential for other Google Apps that have more direct "offline" analogies: GMail and Google Docs. The choosing to go offline thing would be silly for either of these.
With a Gears enhanced GMail, I would expect to always have a copy of (at least) my most recent email. I'd expect to be able to search it, too, but that's another story for another day. For Google Docs, I would at least expect to have an offline version of all my "active" docs. For both of these apps, I would expect to have these things all the time, seamlessly. Anything else is of very limited value in my opinion.
So I am left to assume that Google will enable this kind of seamless enhancement via Gears, but they just haven't figured it out, yet. I think that makes the most sense. If they had figured it out, I'm sure they would have enabled it with Reader, even though it's not quite as essential for Reader. In fact I'll wager the reason they launched with Reader was exactly because they didn't have the above use cases solved, yet.
One other Gear related thing. I was fascinated that Adobe seemed to be hanging around for the launch of Gears. It's interesting in a lot of ways. First, Adobe already has something similar, but far less powerful than Gears: Flash Shared Objects. Gears and its database make SO's look silly. Second, doesn't a Gears enabled Flex application provide everything that an Apollo application could promise? Sure, an Apollo app could be "heavier", have a fancier UI maybe, do more with local files, etc. Finally, I got the feeling that there was a Adobe+Gears announcement that didn't happen. That something that was supposed to be ready for Google Developer Day, wasn't ready. That's why Adobe was there.