I've actually had this for about a month now, I'm just writing about it to distract myself from The Sinus Infection From Hell. It's been a good (and wipe-clean!) companion through that.
The relevant technical specifications are... Model: PRS-300SC, Memory: 512Mb, Display: 5" / 600x800 / 200dpi / 8 grayscale.
The device is about half an inch shorter than a regular paperback and half again as heavy, which is about 220g versus 150g. At times it can feel heavier than books that're heavier than it, since being thin the temptation is to use it with one hand most of the time. Two's more comfortable. Ironically it'd be more comfortable to hold if it were as thick as a paperback.
Other things it's worth being aware of are that more pressure is required from fingers to skip pages than with the wood pulp equivalent, the background is very noticeably grey when the metal frame is at an angle that reflects light or under artificial light, and frankly it'd be nice (and intuitive) to be able to squeeze the side of the reader you were holding to move forwards or backwards.
If all that sounds like an indictment, it isn't, just noting that ebook readers have a lot of competition from dead trees when it comes to comfort and convenience (even if not shelf space.) Given a few more years to develop lighter batteries, better contrast, cheaper prices, more responsive controls, more reasonably priced back-catalogue content from publishers, etc. and I think they'll become very popular at a price point closer to £50. The PRS-300 Pocket is already far better than using a harsh laptop screen if your main interest is text, and when curled up in bed it means having to move less than to flip pages — it also means you can read more comfortably whilst eating, enlarge text if you're tired, etc. I generally use Calibre (far superior to Sony's own software) to convert documents to ePub format. There's not much point in turning the display to read horizontally, unless you need giant text (in which case consider the PRS-900.)
Being a pocket edition with no touchscreen, it's geared towards pure reading and there's no facility to search text. The absence of a touch-screen also means the viewing area is less reflective, and that there's less drain on the battery. Controls (or rather the screen refreshes) aren't particularly responsive — but you quickly get into the habit of clicking to move on a page as your eyes get to the area of screen containing the last few lines. If I remember correctly I got about seven books out of the residual charge the reader turned up in the mail with, which showed full bars then and which was turned on and off frequently over a couple of weeks... once the bars start going down charge seems to be lost more quickly. At which point you're faced with the hostile reality of something you can't continue reading and really need to leave for a few hours to obtain a full recharge. Better to recharge when the indicator is down to halfway, really. I'd guess that replacement batteries suitable for the 300 will show up on eBay in due course, as ones for the 500 series already have, but I'll cross that bridge when this one starts failing to hold charge.
A soft case is supplied, but no power adaptor (probably as much to make the packaging look svelte as to save costs.) You'll probably want a hard case for travelling, and generic adapters designed for the Sony PSP are available for a quid or two on eBay.
Should publishers be worried? Well, market research suggests that ebook-reader owners purchase plenty of content, on average. I got one because I have a lot of out-of-print books as text files. Then again, I'm also part of the secondary reader base that encourages people to buy books because there'll be a ready market for pre-owned copies... and as a rule of thumb, if I'm going to re-read, re-watch or keep listening to something, I'll pick up a shelf copy. I suspect that in time I'll also chew through more fan-fiction and online writing projects, and may experiment with formatting webcomics into PDF. There are also various classics such as Sherlock Holmes (and others referred to by Alan Moore's series The League of Extraordinary Gentlemen) that I intend to check out. A quick recommendation is King Solomon's Mines featuring Allan Quartermain by Henry Rider Haggard, and I keep being reminded that at some point I'd like to properly format and give a cover to my generation's sadly unpublished Adrian-Mole-beating Gathering Daisies (in Black Latex Caves) by Iain LM Hotchkies. There's a lot more than beach-read bestseller chart drek out there and a great deal of it's legally free, notwithstanding the efforts of Disney and others to make copyright indefinite.
A couple of other reviews I found useful can be found here and here (the latter page is in Dutch, but interesting for the comparison photos showing how less reflective the screen is on the 300 vs the 600.)
As far as I can tell Amazon don't stock competitors to their Kindle eBook reader, though they do allow Marketplace listings for them. Standard price elsewhere is about £140 at the moment, or you can get one down to as low as £100 if you lurk on eBay for a while.
Addendum: there's also third-party firmware such as PRS+ and eBookApps that extend functionality (and the latter can be soft-loaded; don't know about the first, since it's still being developed for the PRS-300.) Both seem buggy at the moment, but what might be worth a look if you're technically inclined is this firmware from a Russian coder that allows user styles to be applied to ePub books, as well as set some of the numbered keys to function as navigation ones, and a clock. All optional, of course. Personally I prefer to read in DejaVu Sans so I have that set to override the default sans-serif and serif fonts on the reader.
The easiest way to set a custom font, of course, is just to convert your existing files and embed it using Calibre before loading books onto the device. Don't mess with custom firmware if you're not comfortable doing so, because removing something like boroda's isn't as simple as doing a soft reset then holding the down arrow and 0 key.