The tap water is drinkable I would say everywhere on the route. The only springs we avoided were those saying "non potable" (can't remember the exact spelling), and we did not have any tummy trouble drinking from the springs/fountains. I would not bother to take purifying tablets or filters and if you are concerned you can buy bottled water in nearly all the villages.
Water is fresh and cold at the public fountains at the beginning and the end of the Camino Frances. Mid-Meseta, the water quality may be questionable. There is actually a hand pump in Boadilla del Camino for the pilgrim consumption; water is only flowing there after you personally pump it. I have surveyed Spanish water quality pages, and there are numerous towns which provide untreated water. It is considered potable water by the government, but they wish to improve the quality by 'eventually' providing treatment. Locals get used to the local bugs, tourists may encounter the 'touristas'. If you have a strong immune system, not to worry. If you have a sensitive constitution, perhaps you would prefer bottled water in the dry midlands. If you are in the habbit of putting the pilgrim well water in your hydration bladder or water bottle, make a habbit of cleaning it out once a week. Put a shot of aguardiente in your container at night, fill with water, and shake it up. Dump it out in the AM and fill with fresh water. Walk West.
For those of you that do not know, 'aguardiente' looks like water but is not. It has about 95% alcohol :wink:
To add to the comments above, I think most of the water in the streams is perfectly drinkable. You have the odd fountain in the city squares that may have some bad water, especially in the weekend after partying has been going on.
On the Frances you should have no trouble. My advice is to carry on more half litre thanyou think you will need, stash it away so you don't use it by accident. Each time you reach a reliable source marked as potable throw away what you last got and refill. This way, any dodgy water is got rid of. In the heat slightly dodgy becomes very dodgy quickly.