A few points:-
1. I've got a Kindle, think it's a great device, I read a lot, but also listen to audiobooks - I'm not about to suggest that everybody should rush out and get one - and I have no real argument against the "what's wrong with normal books?" perspective. I just like the ability to store many books on such a relatively small device, and some of the other features it has.
2. The text-to-speech works surprisingly well - at least in terms of pronunciation - very similar to text-to-speech on sat navs - but manages to sound surprisingly natural on that front. It's more the tempo and natural pauses that maybe still need some work in the logic - seems to get commas, full stops and paragraph breaks, but still seems to run some things quickly - it's this aspect that can make it sound unnatural, as opposed to the sound of the voice.
3. I've got many ebooks, haven't yet paid a penny for them ;-) But seriously, there are plenty available for free. New releases or books with commercial interests in them, though, and you're either going to have to pay for them (probably DRM'd) or "download" them.
4.
calibre is great, freeware software for ebook readers - for conversion of ebook formats (as long as they're not DRM'd), populating metadata, organising, and transferring to ebook readers. Calibre "knows" the Kindle as a supported device, and is great for managing and converting content to go on it.
5. Kindle doesn't natively support epub (although Calibre will convert them). It does support pdf, txt and several other formats. mobi seems the preferred format. Text-to-speech didn't work on any of my pdfs, but once converted to mobi, does.
6. The screen isn't backlit. It is e-ink, so is effectively always on (no current required, so I understand, only for changes). So if you want to read at night, you'll need to do the same as for a normal, physical book - book light, or lighted room. Amazon sell some leather cases with integrated lights (white LED - that use the Kindle's battery - can't imagine they'd make good conversions to bike lights, though ;-)) - they are pretty pricey, though - around the £50 mark, although are clearly a Kindle product and fully integrate with the Kindle. A good case is worthwhile, they are thin devices, and made of plastic. Not touchscreen, but a screen protector is probably a good idea. The screen is easily viewed with reasonable amounts of ambient light, if you've not seen an e-ink display, before, you'll probably notice how clear it looks compared with normal displays on similar types of devices. One other thing that bowled me a googly - when you get a Kindle out of it's box for the first time, it has some protective covers on, once you remove them, there's some setup instructions on-screen - and I was looking for some other screen covering to remove - it's the e-ink display, as soon as it's powered on, it will change.
7. Audio books are supported, so can be bought (probably DRM'd) from audible, or ebooks (probably DRM'd) from Amazon. I've got plenty of audiobooks (mp3 format), but haven't yet tried them on my Kindle, yet. There's a folder structure for audible audiobooks, but there's normal mp3 support (haven't checked about playlists / m3u support, though) so mp3 audiobooks should be OK using mp3 support.
8.The 3G option is free-for-life once paid for (makes the Kindle price around £150), the normal Kindle being WiFi. Books / content can be transferred via USB, too. I guess 3G is useful for those that want new content when they're out and about - say new newspapers or magazine content whilst they're commuting on a train, for example. Otherwise, if that sort of thing is not important, then the normal / WiFi one is probably good enough. I suppose it depends on your needs / wants - the price difference doesn't seem very much for lifetime 3G access - and there is a (experimental) browser on the Kindle - although I've never tried using it. I've setup WiFi on mine just to get it synced with my Amazon account, but after that WiFi has been turned off, and all the content I've added has been done using USB.
9. Features like being able to add annotations / comments / notes, and bookmarks to books you read are touted. I guess it depends on the type of reading you do. Personally, I've just used it so far for reading and bookmarks, rather than anything more in terms of customising or adding to the content.