I live in NL (expat from US) and live in Haarlem (near Amsterdam). I don't know Leiden but the best part of cycling here are the miles and mile (or kilometers...) of bike paths. That's the most frustrating part of riding in the US (midwest) is fighting with cars!
It is windy, and you won't find much elevation. But you can get everywhere on your bike, and if you want to dodge the wind, there are plenty of paths for cyclists that are more covered. Rode to the beach yesterday and it was great (windy but great!!).
There are cobblestones but you can also avoid the uncomfortable ones. Anyone who cannot find a good path here is not looking...I also have my hybrid (for bumpier rides) and my race bike (for the nice smooth rides where I want to do alot of mileage).
Good luck in NL, and have some great rides!!