XKCD needs no introduction. It's the first Google search result for "webcomic," and its only real contender in terms of stick-figure-comic popularity is Cyanide and Happiness. You certainly already know about it, and friends who don't read webcomics will send you links to strips they liked over email.
I don't read XKCD regularly anymore, though. It's still clever, and I still enjoy it when I read it. But despite its math-nerd relationship humor and insightful musings, it's not a terribly deep comic. And I miss its experimentation: the experimentation with contrasting art styles, with the fourth wall, with parody (and that A Softer World parody is spot-on, right down to the art and the alt-text). Between its present simplistic art and lack of ongoing plot/complex characters, if I did read XKCD regularly, it would fill the "quick read" category, and my quick-read needs are currently met by The Book of Biff and Thinkin' Lincoln.
XKCD is certainly not a bad comic, and I'd absolutely agree that it's good at what it does. But there are a few comics out there that, in my estimation, do the same things that XKCD does, but better. Let me share some of them with you.
Piled Higher and Deeper
Jorge Cham's Piled Higher and Deeper--PHD--is a little comic about getting your doctorate, and it has a number of things in common with XKCD. They both feature nerdy scientific charts. They both revel in academic esoterica. But there are two things PHD has that XKCD doesn't: sharp, colorful art, and a diverse cast of characters who learn and grow through their experiences of enduring higher-education bullcrap. You've got engineering nerd and chocoholic Cecilia, student activist Tajel, expert procrastinator Mike Slackenerny, and the strip's unnamed protagonist. PHD is smart, funny, and surprisingly relatable--I enjoy it, and I'm not even in grad school.
Rock, Paper, Cynic
Ever wish XKCD weren't so darn optimistic? Then Rock Paper Cynic is the comic for you. The creator, Peter Chiykowski, openly acknowledges a creative debt to XKCD, but unlike his inspiration, he goes for the throat with unmatched relentlessness. From morbid twists, to nerdy cynicism, to raw angry vulnerability, RPC delivers a caustic and critical edge. It's got your intellectual esoterica, and it's even got its own twisted brand of optimism.
It's creatively acerbic, and it updates daily. Decidedly not for the childrens, though--it can get pretty dark in there.
So you like quick gags, few recurring characters with zero character development, and sketchy black-and-white art? Check out Chainsawsuit. Granted, it's not as intellectual as XKCD, but it's still darn funny. It specializes in unexpected twist endings, abject silliness, and juvenile gross-out humor--sometimes all three at once. In my opinion, Chainsawsuit shines hardest when it's parodying: as with Disastorm, the extended disaster-movie satire. Chainsawsuit even parodies XKCD!
To conclude: do you read XKCD regularly? If so, good. Keep on reading it. But if you like XKCD, chances are you'll also like these comics, so give 'em a look. They're good stuff.