I have a PRS-600, which I think is more or less the same as a PRS-650.
Super device, lasts about a week on a charge, easy to read, and will handle just about every eBook format except for Mobipocket (which is pretty much defunct now) or Kindle (which just means I don't buy my ebooks from a certain south American aquatic source). On long overseas trips, I can buy and download new books easily also, which is great when running out of reading matter in non English speaking countries (and saves a lot on baggage allowances).
Handles MSWord/RTF files very well, PDFs variable, and Adobe ePub fantastically well. With a standard SD card in the slot, holds about 200 books (or equivalent).
It works in a cockpit fairly well, although I've not yet tried loading approach plates and checklists onto it - an experiment to be had eventually. No RF/bluetooth/Wifi/etc. connection which is more help than hindrance in the air - you know it can't possibly try and interfere with anything - it charges and connects to your PC on a USB. Also not having its own Wifi/3G it can't have a mind of its own either - nobody controls it but me.
The PC software provided for moving books back and forth is absolutely horrible, but works, just.
My main whinge about it is the variability of pdf handling - some work really well, but scientific conference papers (which for my sins I have to read a lot of) with figures, graphs, diagrams in can often be a real struggle - and on one occasion it just crashed altogether at a really tough one.
Also useful for taking meeting notes, and fits in a jacket pocket very well.
For those who like such things, it's also an MP3 player, although I've never used it as such.
There are a lot of independent eBook retailers nowadays - although it pays to shop around since they aren't consistent in their pricing. You can usually get the latest bestsellers easily, and then a randomish spread of older stuff. Project Gutenberg is brilliant for a near-infinite selection of free out-of-copyright books.
If you just want something iPad-ish, without paying an obscene amount of money to Apple, go onto ebay and search on the phrase "Android Tablet" - you'll get a range of nice little devices, using a Google operating system, connecting as required via USB or Wifi, and lasting 3-4 hours on a charge, that work really well. You can download a free Kindle reader for those, and they'll take a mouse and keyboard via USB, which an iPad won't, and nor will any ebook device. However, the big disadvantage (to me) is the 3-4 hour battery life, versus the 7-10 day battery life of my Sony, and presumably the Kindle and other similar devices.
G