I'm a newbie webcartoonist (by which I mean I've only been doing this for 9 mo. or so), so I have a somewhat different perspective from the Joy of Tech folks. I can't help but like the iComic app, if only for its simple elegance (although, out of respect for the wishes of other webcartoonists, I don't use it to browse any webcomic besides my own...it's a handy means of flipping through my own archives).
I liked it so much, in fact, that I wrote the plugin for my comic and shared it with the other iComickers; doing so helped me address some of the concerns raised above and got me a few new readers in the process. I was able to streamline the process of retrieving and downloading the strips so that it would take no more bandwidth than going through the archives by means of the web site (sometimes it takes less bandwidth, in fact) and wouldn't generate any error messages. I was also able to put in links to my main page, forums, and so on (though I don't know how many people actually inspect that little drop-down menu of links).
It could be argued that those of my readers who use iComic to view the comic "aren't doing me any good" (for lack of a better phrase), in that if I had advertising or a store on my website, they'd never see it. But, the way I figure it, perhaps the iComickers will come through by telling their non-iComicking friends about my strip and my readership will grow.
All that said, I do understand the concerns raised by Snaggy, and they're valid ones. I think maybe RSS will provide an alternative that's still convenient for readers, but allows greater interaction between the creator and his audience.
So I've got one of those, too. ;)