The first time I was impressed by on-line mapping was when multimap.co.uk added a feature where they'd alpha-blend street maps onto aerial photos. You could pan a rectangle around the aerial photo, and within the rectangle you'd see the map composited. For civilians like me, it was a great improvement on aerial photos, and more interesting than a plain old street map.
I had two complaints, though.
One complaint was that I couldn't pan the map; if I hit the edges, it didn't start to auto-scroll. It just clipped my compositing rectangle. If I wanted to pan the map, I had to click an edge, and watch the page reload! Not just the images in the page; the whole page! Really distracting, and an awful way to follow a street.
My other complaint was that it was using about 500 square pixels of my 23" display. No matter how big I made the browser window, it filled the same area with map data. It didn't even use the excess to show me more ads; it just left it blank. [Admittedly, since then they've added a "Bigger" button, but it only doubles the width to give you something like 700x400.]
Google don't let you blend with aerial photos yet, but I'm sure they will. They have the data, after all. And that feature always seemed more interesting than strictly useful. The worst problem is that Google's maps only cover North America. A shame, because the British Ordnance Survey makes really nice maps (and not just for the purposes of moving artillery).
Oh, and it doesn't work with Safari yet. Camino works fine.
I'm still waiting for somewhere that will let me blend, say, Bracknell (Berkshire, GB) and San Jose (California, US) to get an idea of their relative sizes. It's not hard if you've got the data.