Since the overlays are limited to 3mb it's immaterial really, but for the sake of trying to give a better picture of how I create routes...
In google maps I would typically have a bookmark for a template route that is a circular with a say 4 via points that force it around say a 100 mile loop. Then when I create a specific route I go to say my west template and move the via points about and add new ones. Typically I will go right up to the maximum 23 via points.
On google maps a via point is a very distinct white circle that does not scale. When you drag a via point you will occasionally during the draft stage get 'cul de sac routing' where the route goes out and back on itself. Adding further via points corrects this and forces the route around a loop. Google routing algorithm seems to generate less culdesacs.
At present I use an electronic OS map for reference on one screen and route on another with google maps. I know exactly what roads I want to use, and am not interested in the route suggestions that 'follow road' gives, I use follow road for speed since I want to follow the road as opposed to route freestyle, though this is useful to toggle on/off so I can use the little footbridge over a river, and I still want to route as per car, since frankly cycle auto routing is a long way off being honed as has been argued at length on here. So speed and the embedding of turn/course points that with the right service can materialise in the generated TCX/GPX.
A Saturday ride for me will typically utilise 140 miles of the narrowest surfaced roads, so the narrow gauge yellow (minor) roads on OS. I will look at the OS and see a route that keeps to these as much as possible - it's just what floats my boat. I have even experimented with putting OS maps through photoshop to filter the narrow yellow roads making them stand out to enhance seeing the wood for the trees.
So then it is a process of looking at each screen to quickly build a route. Depending how fussy I am being I can knock a quality route out in as little as 30 mins.
I also use the google street view feature to quickly check navigable roads. Drag the man without dropping the man and all the streetviewed roads are highlighted, so you can see at a glance if your route is actually a bridleway. Not that I mind the odd bit of that.
Event I organised using one of my routes: http://vimeo.com/user16410237/hom2
I can't stress enough how vital the behaviour of the via point is to the way I create routes. On basecamp they are practically invisible and I will be dragging all the via points about constantly gradually moulding the route. And I'm amazed how slow it is in basecamp as opposed to a browser restricted google maps.
I wondered if I could do it all in google earth, but alas there is no via point capability yet. I think the overlays in google earth pro would make the use of geo referenced map formats a breeze, but £270pa is too rich for me!!
Since I'm a developer, I might even spin my own solution, though i don't really have the time.