I visited this WHS as part of a trip to Rotterdam today. Like others, I had explored the option of taking a guided tour of the building, but unfortunately this didn't work out. At this time of year (February), and maybe at other times too, the guided tours only take place at the weekend, by which point I would have already left the city.
However for those interested, there is an alternative to booking an expensive tour with Urban Guides. Obviously, having not experienced either option myself, I cannot vouch for how good it is, but the Chabot Museum offers tours too. From the Chabot Museum website there are booking options for several tours per day (on Saturday and Sunday), each with a cost of EUR15 per person. Included in the cost (from 9th February onwards) is a shuttle bus service from Chabot Museum to the Van Nellefabriek site and entrance to the Museum itself.
If like me, you have to visit without a guide, I was quite surprised at how much I got to see. It is possible to reach the site with public transport from Rotterdam Centraal train station, either with the Number 38 bus, or via the Number 8 tram (alight at the last stop). I took the tram, and from the last stop it is a 0.7 mile walk to the factory site.
There is a security hut at the entrance but there was nothing stopping us from walking in, so we did. The guard saw us, but did not stop or even speak to us. So we were able to wander freely around the exterior of the factory and gaze in through some lower floor windows.
I do like this type of site, and could appreciate in 'laymans' terms the effect that the architecture would have had on those working in the building and the conditions in which they did so. I particularly liked the glass bridges linking the sides of the factory and thought that these looked a lot like the footbridges leading onto cruise boats, creating a nice link with the maritime history of the city.