I've got a mini, mainly purchased to display and read PDFs for school, which it excels at - in fact I've got some double page scans to read for class tonight, which I'll do on my mini (with much joy compared to printing out reams of paper). Personally, I've not experienced any of the negative effects of reading on this device, despite the fact my eyes are fast approaching half a century old (in a few short years, sigh). GoodReader is a great PDF app and allows for cropping excess white space on pages, which maximises the viewing of actual text which might be one reason my experience has been nothing but positive.
As for reading other books that aren't PDFs, most of the time I choose my Kindle over the iPad, in fact I just upgraded my bog standard Kindle to the Paperwhite, which is awesome. I'll never give up my Kindle, reading on it is too much fun but it's a complement to my mini (about which I can't say enough wonderful things) which does so many other things (especially the games!).