Dharmesh Shah, Co-Founder and CTO of Hubspot and MIT alum, has a really excellent blog post on the whole topic of NDAs which is worth reading in its entirety. In this post he recommends that startups steer away from signing NDAs. Here is a summary of his advice:
"1. It's often in your interests to share the “what” (but not details of the how) as early in the process as possible. In this case, an NDA is not relevant.
2. Don’t rely on an NDA to protect the truly “secret” stuff. It’s likely not going to give you the protection you think it is.
3. Your odds of actually getting someone to sign the NDA are low anyway, so it’s likely not worth the effort."