David's meditation on the powers of God makes this psalm one of my personal favorites.
Consider the relation between Creator and creation! David rhapsodizes about how God knows him intimately. Everything - from his actions to his thoughts to the words he is about to say - is known by God. This familiarity with David even took place while he was still in the womb (vs. 13-16). God saw him while he was being formed, and God knew all about his days before even one of them came to be!
The Lord is also omnipresent, all around David (and us). He hems us in, being both behind and before us (vs. 5). David asks rhetorically, "Where can I go from your Spirit? Where can I flee from your presence?" (vs. 7) knowing full well that the answer is "no where." No matter where David goes or tries to flee from God, it is hopeless. God will be there. (Jonah could have used that lesson!)
Psalm 139 is really a love song that David composed for his Creator. And it is one we can join along in and sing with our souls, too, because these truths apply to us also. God's ways are amazing!