Just came back from Amsterdam. Was in Keukenhof on 30 April. Things you'll need to know:
1. the VVV Amsterdam Tourist Information office (operated just opposite the Centraal Station in city centre) offers a joint ticket at the cost of Euro19.00, including the admission ticket (@13.50 Euro) and the bus connection from the city to the Garden). I didn't do this though, because I wasn't sure if I will definitely go.
2. I saw someone said it's over crowded with many tourist buses. Maybe he/she's got a point, but I went in the afternoon, at around 3pm, and it was not too crowded. By the time I left at 6pm, it was very tranquil and peaceful. So maybe you can consider going later in the day to avoid the crowd.
3. "how many tulips can I see?" I can see that the owner of this place tries to give the flowers (not just tulips but majority are) different times to bloom so the garden can justify to operate in a span of two months every year. At the time I was there, IT WAS BRILLIANTLY BEAUTIFUL! 90% of the flowers are either yet to blossom (20% of them) or are just blossomy now (80% of them), only 10% of the total is already gone.
4. It's not a big place -- if u just walk around the whole place without stopping it'd probably take about 1hr or so, but if you really like the flowers you'll find evne 3 hrs are not enough!
5. the reasons I'm giving only 4 stars instead of 5 stars are 1) public transportation wise it's not very convinient and 2) Euro 13.50 is bit too expensive.
This review is the subjective opinion of a TripAdvisor member and not of TripAdvisor LLC.