The truth is, if the LinkedIn crack had happened a few years ago, I would have been in non-trivial trouble: turns out that my password (now changed, of course) was one that I'd used at a lot of sites. It was consciously my "yeah, whatever" low-security password that I was using for sites that I didn't think were terribly important -- but some of those sites have become more important to me over time.
But the nice thing about LastPass is the way it has changed my habits. It doesn't just keep all of my passwords in a nicely secure locker (hidden behind one mnemonic-to-me but *really* hard to guess password); it also integrates so well with my browsers that it's really easy to *always* use secure passwords. When confronted by a new site, it offers to generate a reasonably high-security random password, and then creates a new record to keep track of that password afterwards.
The result, yes, is that I don't actually *know* most of my passwords. But I don't need to, so long as I have either *some* kind of Internet access, or a machine that has my password locker and LastPass loaded onto it -- which is very nearly all the time. And it means that no two sites have the same password, so even if one site is compromised, the rest of my online identity is still decently safe.
This is an unpaid plug: my only connection to LastPass is as a customer who wants to see the company safe and secure, because they are providing me with an invaluable service. And frankly, at only $12/year for the "premium" service, I consider it well worthwhile to subscribe at that level, even though the free basic service provides most of the important features. IMO it's one of the best bangs for the buck that you can find on the Internet today, and I encourage you to check it out...