How seriously do colleges take National Merit Semifinalists?

While yes, it’s a nice thing to have on your application, i don’t think it makes a big difference. Most kids who ended up as semi-finalists are strong candidates anyway, and the colleges get to see what they need of an applicant, so the assessment of NMF doesn’t have a big impact on the decision. It’s really hard to tease this out of the rest of the package, though, so that’s just my opinion.