The NKJV is right there with the NASB as the best translations out there. If you lean towards majority text in your beliefs, then the NKJV would be for you, if you are more of an Alexandrian then the NASB. I personally love both and use both a lot, though I lean slightly to the NASB.
But specifically to your question, the NKJV is much more readable and understandable for today's student of the Scriptures than the outdated language of the KJV. It also is just plain a better translation. My personal view of the best translations is this.
1. NASB/NKJV.
2. (very close to 1) ESV, NET.
3. (a little more distant) KJV
4. (a lot more distant) NIV
5. (comentaries, NOT translations) NLT, Message, et al.
I don't have real experience or much exposure yet to the HCSB or the NRSV so I did not rank those.