Market apps will update as needed.
Navigation works as well on the Eris as any other android phone. The only complaints I've seen tend to be Google related (missing street, one way streets routed the wrong way, etc) rather than Eris related. (I've also seen 'GPS can't find me" type threads, but those are a problem with that specific phone, not the Eris in general). I personally have used it for some 500 - 800 mile trips and several 50 mile or so ones with no problems. Navigation does drain the battery, but that's going to be true of any phone, just keep a car charger handy.
The Eris can be rooted, which gives you the ability to 'update' the OS to something you like better. Many of us are running ROMs based on Android 2.2, whereas the stock Eris is on the 2.1 version of the OS.
