I've cycled in NL (mainly The Hague and Amsterdam) and in Brussels. In that order they are good, safe but a pain, and not much better than Basingstoke*.
I like the absence of helmets in all three, and the cycle roads and lanes are good in NL. Unlike the UK ones the surfaces support speeds over 5mph, and the broken glass is less than 10mm deep.
The general attitude to cycling is different. No-one thinks you're mad or cranky for cycling, few view cycling as dangerous. it's normal and mainstream. Drivers are just as aggressive as in the UK but cyclist specific aggression isn't as common.
That said I enjoy cycling in the UK and don't find the problems some here do. When in London I now find it a most enjoyable place to ride. Some of the arterial roads in and out of central London aren't cycle friendly. I don't use the A3 from Hook to Richmond for example. The same applies anywhere else including The Netherlands. 3 lane dual carriageways aren't nice places to ride. There are always other roads.
*I rode a bike in Basingstoke last week. Can I reconsider the A3 (as above). I'm nominating Basingstoke as the least cycle friendly place in the known universe.