Thought I would post my recent experience for anyone looking....
We were in the Netherlands and Belgium for a week recently. I have an unlocked phone that I take and have had SIM cards already in London and in Germany (with some roaming in Czech Republic)
Compared to the other countries, the Netherlands was a breeze, just popped into a Vodaphone store (I prefer to go to a dedicated carrier store where they will make sure it works before I leave, rather than a random Kiosk) laid down 10 euro, no real registration, and in a couple minutes walked out with a Gig of data. Due to changes in EU law, I also used the same SIM in Belgium with no elevated roaming fees.
Coverage was good, but did find some spots where coverage went from 4G to "G" to "E".
I do hear that Germany has greatly complicated registration since I was there a year ago, but this trip was a breeze. The next try will be Madrid in February. I did consider that with easy roaming within the EU, I might find a SIM to load, but keep active and top up when I plan to travel, but have not investigated the ins and outs.