Hi, I cycled from Trondheim in Norway down to Santiago de Compostela last year on Eurovelo 3 Pilgrims Route (5500km by the time I got to Porto in Portugal). Eurovelo signage on this route was patchy to say the least (more like non-existent!). However, in some countries you are following established national cycleways and therefore you can follow the local signs. I pieced together a route by researching national cycleways and stitched them together into a trans-national route.
The Eurovelo project is more a grand vision than a local reality - like many things EU-related! Some of the routes are more established than others - Eurovelo 6 for instance from the Atlantic to the Black Sea has Eurovelo signage most of the way from what I understand together with maps and a route book (Bikeline series by Esterbauer Verlag). Eurovelo 8 has little if no signage. Depends on which one you are thinking about...
Michelin maps 1:250,000 and 200,000 are a good bet for route planning. I certainly used them in Belgium, France, Spain and Portugal. I used Freytag & Berndt maps for Scandinavia and Germany.
For the most part I camped along the way. I travelled through Norway, Sweden, Denmark, Germany, Belgium, France, Spain and Portugal and only away from the coast in the latter two countries was it more problematic. I've toured before in Spain and Portugal and found that away from the more popular tourist destinations camping can be a bit tricky. But hotel/hostel accommodation in these countries is a lot more reasonable than in Scandinavia!
Take a look at this guy's website. He's done 3 of the Eurovelo routes and it paints a pretty realistic experience..
http://cyclingeurope.orgHope this helps. Let me know if I can help further.
Andrew