It's a good formal equivalence translation faithful to its textual base (the modern UBS Greek text), but in some places it departs slightly from formal equivalence. There is, however, a better formal equivalence translation from the same text base, the NASB. Another better formal equivalence translation, although from a different text base (the Received Text) is the NKJV, which is my preferred Bible.
That being said, I think the ESV is a very good choice for anyone looking for an accurate, understandable modern translation.