I bought my Sony PRS-600 at launch back in August direct from Sony Style. I went for it over the much cheaper PRS-300 because of its larger screen and memory card slots - it supports both SDHC cards and Sony Memory Stick cards.
With that said, I still have about 256MB free in the internal memory without having inserted a memory card yet.

I find e-ink screens like the one used in the PRS-600 to be far easier on the eye when reading for extended periods than any screen technology that requires a backlight. Battery life is excellent. I read for an hour or two a day on this device and only have to charge it once every two to three weeks, depending on my usage. It charges from my computer's USB port. Be aware that you cannot plug this into any wall-plug type USB charger, as it requires a driver on the computer to begin charging. Sony do offer an optional wall charger for it, but it doesn't use the USB port. If you've got a Sony PSP, its charger will work just fine with the PRS-600.

They include a neoprene sleeve in the box with the PRS-600, but I soon replaced it with a clip-on leather cover, which does a good job of protecting the screen when the reader is not in use.

I have read about 30 books on this device at this point, and love it to bits. I travel a lot, and would not have been able to keep all of those books after I read them if they were in paper form. I mostly read novels and history books on it, and it is great for that purpose. I'm not sure how well it would do with something like technical manuals.

The PRS-600 also has a jpeg viewer, which is pretty pointless considering the grayscale e-ink screen. However, it can play MP3's (which the PRS-300 cannot), so could come in useful for audiobooks, although I prefer my iPod for that kind of functionality. As far as I'm aware, it is not supported by Audible.com right now.

The only real issues I have with the device, as with most Sony products I've tried, is the weak software they supply. I find it sometimes leaves duplicate copies of books on my device. However, you don't actually have to use the Sony software unless you are buying books from their store. The reader appears as a removable drive in Windows when you connect the USB cable. You can drag and drop files (epub, pdf, txt, etc) into it and they will be available on the device. You can also use the excellent open-source software, Calibre, with the reader. It can manage the books on your device, convert between formats, etc.

I have also bought ebooks from sources other than Sony's store (booksonboard.com, for example) and they worked fine, although they did require launching the Sony app (or Adobe Digital Editions) to make their DRM work with the device.

I think the most persuasive thing I can say about the reader is this: I haven't bought a paper book since I got the PRS-600.