Just trying to get my new 60CSx together and have downloaded some topo maps here, but wondered if I need to buy Garmin's City Navigator or is there an alternative available?
If you want full address search and routing for the whole US, there isn't any alternative. There are some free OSM routable maps, but they don't support address search and there are routing quirks. I don't know if it would even be possible to load the whole US into your GPS this way.
Garmin's 24k topo maps also contain the Navteq address and routing data, but each product only covers a few states and costs about $100.
For accurate driving directions I'd recommend City Navigator. It's around $75 at Amazon. With OSM maps for the US you are relying on volunteers to verify the accuracy of a Tiger shapefile import. Those files weren't originally intended to be routable, so there are a lot of bugs in them.
If you're driving on the roads get City Navigator. I tried using the OSM routing maps on a recent trip to DC on my Dakota 20. Although they worked it was hard to use while driving since it didn't have like Exit at exist 20 and just said exit in 0.2 miles which was hard when there are 2 exits right there.
You could always try the free OSM routable maps and if they don't work well for you then pick up City Navigator.