It works exactly as Matt said, and was VERY easy for me.
I picked up a copy of Metroguide v7 on ebay for $30 shipped, paid $6 for Metrogold, and the turn by turn function works great on my Vista HCx. It's almost as good as my Magellan Crossover, but of course the HCx doesn't announce upcoming street names, etc, like the Crossover, so is not quite as good as a dedicated car unit [but it's WAY better than the Crossover out of the car!].
I think the Metroguide maps [which include POIs, etc] overlay my Garmin Topo maps, but am not quite sure. What I DO know is that I can do a batch upload of caches to the HCx using the premium membership features at geocaching.com. So now I can get turn by turn directions to a cache from my home, or if I happen to see a cache coming along a route I'm on, I can select it, and get turn by turn right to the cache. It's cool, and exactly what I was wanting. Only downside is that the maps are a bit older than City Nav, but that's not a big deal to me. Upside is that it's cheaper than City Nav, and you don't have to mess with the all the unlock drama.
Count me satisfied!