I use a Kindle 3 (keyboard) and the Kindle app on my iPad (and iPhone). My books (purchased at Amazon, download or shared from
http://lendle.me ) all stay synced between devices on the same page.
The Kindle I keep in the car (and use to read in bright light) and the iPad at home (usually in bed with backlight dimmed and lights off) the iPhone is a backup in case I forgot (or didn't think I'd need a device that day). But it's awesome to always be on the same page automagically.
FYI, I specifically picked the black and white 3g Kindle 3 (keyboard) model for TWO reasons:
1) the free 3g browsing (any sites, not just the limited site in current 3g models). I don't use it often but when I want it, it's great.
2)
http://kindlefeeder.com sends magazine type formatted news (mostly full RSS feeds) vi e-mail to my Kindle daily (it's an interesting intuitive navigation system if you haven't seen it and it's nicer than the way the Kindle Fire or iPad app). The newer Kindles only use this "magazine" format IF you sync via USB. Very inconvenient! The e-mail way it's just on my Kindle the next time I pick up up; the WiFi reaches the car or if I miss it, I can download via the 3g (or even force generate a new block of news).
3) The black and white works well in the sunlight, not like tablet glare issues.