No, the one I use isn't an RSS reader. I did use RSS for comics for a bit, but it was shit to do it that way. There are a better comic-specific services. The problem with RSS is that it just sends you a link whenever a new comic comes out so you're left managing a ton of RSS posts if you read a few comics. It's also no use for tagging an old comic you'd like to read. What's the point of getting RSS notifications of new comics if I'm 1500 pages behind? So RSS isn't an integrated solution for managing all your webcomic reading. Also, that approach pretty much means you're juggling bookmarks, and that's not a very reliable way to keep track of anything, and is also specific to your specific browser install, so it's not a portable solution.
RSS sends you a new individual post every time a new comic comes out so you have to go and manually remove the posts if X number of comics comes out and you don't have time to read them. So for SMBC which updates like 5 times a week, if I didn't have time to read it for a couple of weeks now i have to wade through 10 posts in my RSS feed. That's more work. The point is to make it less work. I use comic-rocket.com, it's the best one I found so far. You just tag the comics you want to read and it auto-tracks what page you're up to, and also auto-sorts the list so the ones with new comics but that you're *closest* to catching up with are at the top. And importantly, it doesn't spam posts at you every time a new comic come out like having an RSS would. RSS is like being on a treadmill trying to keep up, whereas with what I have set up now I just jump on the one site when I have time, see which comics have new releases and decide which ones I *want* to catch up with.