Text can be read without conversion. HTML and PDF needs to be converted. PDF conversion is frankly iffy but then so is reading unconverted PDF on a 6" screen. Simple text PDFs do pretty well. Tables often go wonky, in my experience. You can convert files with the Mobipocket software or have Amazon do it for you for free. If you want them to send the converted file to your Kindle wirelessly, that's an extra 10 cent charge.
If Sprint EVDO service is not available where you are, you can most definitely read. You can also load new files through the USB like other reading devices. The only things you couldn't do would be to download files or over wireless, shop the store from the Kindle itself (rather than your computer), do a web or Wikipedia search, or use the web browser for any other sort of internet stuff. You could purchase books from Amazon, download them to the computer, and transfer to the Kindle but I don't think you can do that with samples. I may be wrong but I think the free sample chapters only go wirelessly.
The DRM-ed Mobipockets you may wish to buy from other stores can sometimes be read on the Kindle by using
igorsk's hack.
You may also want to look into the Cybook Gen3. It may cover more of the bases for you.