It's pretty easy to submit but it takes them several months to process and approve your first comic. After that first hurdle, the approval process is much faster for subsequent titles.
Worthwhile? I dunno, I actually submit more for the publishing credit than anything related to sales or readership. It just looks good on the resume when people are looking to hire me.
Comixology is a store that sells digital comics. Unless this has changed, you can't upload a webcomic episode or page by page, it has to be a completed book.