I use a Kindle DX (the pearl e-ink one) for reading A4 formatted PDFs. It does the job really well, but I reckon I only really use it for (relatively short) scientific articles. Larger PDF files will indeed add too much load delay. My old Hanlin v3 had some Adobe licensed PDF renderer that did reflow on PDFs, it was sure better than what the Kindle does or what I succeeded doing using Calibre and cmd line tools (the Hanlin v3 sucked in other ways).
Most of my e-ink reading falls in the 'normal book' category for which Mobi files on a Kindle3 suffices. (One killer feature of the kindle for me is being able to read Wikipedia articles while camping abroad about the region I am visiting, it sure beats any 'history section' of regular travel guides.)