We are currently walking from Lisbon, having just reached Santarem. We have found the signing so far to be adequate to extremely good, mainly very good. Many of signs look to be new and combine the blue arrow of the Fatima route with the yellow arrow of the Santiago route. We are also using a combination of books, the
Brierley maps-only guide, (not the full guide) and the CSJ guide from Lisbon to Porto, available for download from this forum's Portuguese Resources section. Those along with many painted arrows in either blue or yellow have been enough. Some of the painted arrows are rather faded in some places, but we haven't stuck any major problem. Obviously we can't speak about places between Santarem and further north, but from what we've seen you should be OK.
Further to the note, above, about the CSJ Guide. That guide is very detailed, and very useful, particularly so for the initial stage from Lisbon to Sacavem, but it follows that by being rather dismissive of the route from Sacavem to Alhandra, describing it as "An ugly route by an old canal", and "an illegal dumping zone". It writes off an 8km stretch in this manner with just over four lines of text. It may not be the prettiest route on the Camino but it is very nice with lots of wild flowers and assorted wildlife along a very nice tidal river valley. We were dreading this stretch from its description, but ultimately found it exceedingly interesting and rather pleasing. You are under the flight path from Lisbon airport a lot of the time but the noise of the birds and frogs made up for that. Also there are places in the guide where the distances are wrong, sometimes wildly (says 3km but It's more like 13), best to check against a map. The sections after Alhandra seem to be much more balanced and useful. We're hoping to submit a update report after completing the Camino.