I currently use TomTom on my PocketPC. The iPhone will do LESS for me (it'll take me longer to do things that I do frequently, and won't even do some things at all), but as soon as I saw it at MWSF, I knew I had to have one.
But, I'm confident most of the issues I have at the moment (which are all small, admittedly) will be addressed in future software updates, and/or I'll find a way to work around them (things taking a bit longer to do isn't really a problem when it's an iPhone you're doing them on!). Issues so far: no search for calendar entries/contacts, no MMS, no copy/paste, limited ringer profiles, calendar not very flexible, no "smarttype" for quick contact finding/dialling etc. As you can see, they're all things that can (and I think mostly WILL) be updated with software updates, maybe before we even see the iPhone in the UK.
So, with this in mind, I'll be getting a standalone TomTom from eBay to tide me over until Google Maps supports external bluetooth GPS receivers for sat-nav guidance, or until there's a 3rd-party iPhone sat-nav solution available. I hope this happens.
All I want to be able to use Google Maps Directions for is to be able to sit in my house (or at work) and see how long it'll take me to get from A to B, and a rough idea of the route, the day/night before my intended journey. This will save me having to get into my car and boot up my TomTom. So it looks like it'll be ok for this.