I have no experience with KNAB, but I do have savings accounts with Nationale-Nederlanden and LeasePlan Bank. I am happy with both. Opening an account is easy in both cases, and so far there has been nothing to complain about.
At the moment NN is tied with KNAB and Argenta for highest variable interest rate (1.65%), and LeasePlan Bank offers the highest interest on a one-year time deposit (1.95%). But thanks to spaarinformatie.nl you already knew that, of course. :)
I will say that KNAB Plus looks like a rip-off at €5/month. KNAB Basis is more reasonable, being free. If you're looking for both a current account and a savings account, then ING + NN would be cheaper than KNAB Plus.