GnuTLS had its own problems recently (see: http://www.gnutls.org/security.html), and regarding NSS... I couldn't find a proper public "security" page for it (eg, release notes for 3.16 point to CVE-2014-1492; but the link to the bugzilla issue is not public), so I don't know.
Obviously a good security record doesn't imply things like a good code base, good practices, etc; (although it could).
Quote: "The cert_TestHostName function in lib/certdb/certdb.c in the certificate-checking implementation in Mozilla Network Security Services (NSS) before 3.16 accepts a wildcard character that is embedded in an internationalized domain name's U-label, which might allow man-in-the-middle attackers to spoof SSL servers via a crafted certificate."
Edit: Added Quote
My point is that having a proper security page is a good practice and I really appreciate it as user.
How much more fun/comfortable is NSS/GnuTLS to use in a typical C project in comparison to OpenSSL?
Great potential to learn for developers and users alike this heartbleed.
Been there, done that. Sad to say, but I now use openssl.