Mobile Safari is slow and crashy, good thing I can replace it with… waitaminute. As far as I know there's no decent replacement for Maps either; it remains to be seen if Apple will permit a direct replacement. If Google doesn't release one themselves, it could be problematic for a third party to create an app using Google's map API - that's centered around use in web apps, although their "Maps for Business" might be suited. Still, it's a hell of a risk to take. (but possibly with a handsome reward)
You can replace Safari with Opera, Chrome, Dolphin, or several others. You can't set them as default but you can replace them. There are plenty of map alternatives too - Open Street Map, TomTom, Google Earth, etc.
Those alternative browsers either use the Safari rendering engine or render offline, so are significantly crippled compared to something like Firefox for Android.
In case you're not aware: Chrome on iOS uses UIWebView, which is Safari without the fast JavaScript engine.
The web interface for Google Maps certainly works on Safari, but it's certainly not as slick or as fast as the app. Maybe this is due to connection speed, not browser speed, but the bottom line is the user experience is worse (though the features are much better).
