It seemed a bit simple/silly at first, until I discovered that it could take the place of my fancy bookmarklets, and can be integrated right into existing browsers.
So, I guess that makes yubnub kind of like a metabookmarklet, really. You can "lj (username)" and "ljfriends (username)" too.
Now if only they made a "My Yubnub"... a customized page of available yubnub searches, yubnubbed content, etc. hm.