The NASB is a good translation of a good text - if you're using the Bible primarily for study, then I'd use the NASB. However, the great downside to the NASB is that since it seeks to be so [almost] absolute in its translation of the Hebrew and Greek, it makes for a difficult read due to the adherence to foreign word order (for us). If you plan on using this Bible in church or at other occasions that require oral reading, I would suggest the ESV (or even HCSB) over it any day. I only own one hardcopy of the NASB and I rarely consult it for this reason. I am an English major, so reading and understanding it is a nonissue for me, but I stumble over the words when read aloud. Additionally, I feel like I'm translating the translation sometimes when I am trying to explain it to others, which was the same thing I had to do with the KJV.
The NKJV relies on the Byzantine text. You'll find many here who advocate the Byzantine tradition - be it the old Textus Receptus or the Majority Text. The NKJV is consistently one of my top versions for study, but again I much prefer the ESV, HCSB, and even sometimes NRSV to it because of the still archaic language.
My personal drawbacks for both of these versions are the same - neither does particularly well in a public setting.
For some better tangibles on translation, I recommend this Lutheran's Pastor and Seminary President's blog. He is in search of a version to replace the NIV11 and he has consider all of the versions you mentioned. He seems to have a higher opinion of the HCSB while he doesn't much care for the ESV. His criticisms of the ESV are valid. However, he also goes into some depth on the NKJV and its issues. I don't think the NASB ever made it on a shortlist because it's probably the most difficult outside of maybe the archaic nature of the KJV.
Link: http://exegete77.wordpress.com/
