It's a cool feature because it makes the papers more finable, more easily navigatable, easier to read online and faster to scroll through. I am also happy for blind people that they can more easily use ArXive with Braille readers now.

(I'm still a fan of printing the PDFs, because I annotate on paper and refer to page numbers, but the HTML feature is in addition to PDF download, not a replacement.)

One thing that still sucks (not ArXiv related though) is reading mathematical formulae on the Kindle - wonder if someone with rendering expertise could have a look into the MOBI format.