Is Colossians 1:15 really saying that Jesus Christ is the "FIRSTBORN of all creation"? (Colossians 1:15)

Yes.

However, some trinitarians insist that the literal "firstborn of all creation" describing Jesus at Col. 1:15 really means "the pre-eminent one over all creation."

Prototokos ("first-born"), literally means "born first" - see Young's Analytical Concordance - or Strong's Exhaustive Concordance. The New Testament in the King James Version and most other trinitarian translations use this meaning throughout. Here are all the instances of prototokos in the NT: Matt. 1:25 (King James only); Luke 2:7; Ro. 8:29; Col. 1:15; Col. 1:18; Heb. 1:5, 6; Heb. 11:28; Heb. 12:23; Rev. 1:5 (compare Col. 1:18). None of them clearly means "pre-eminent". Not only do all of these scriptures that use prototokos have either the certain or the most probable meaning of "first-born," but we rarely (if ever) see any Bible translate them as anything but "first-born" or its literal equivalent except at Col. 1:15-18 where the actual meaning would disprove a trinity concept! A few trinitarian translations force an improper interpretation for prototokos at this scripture only (e.g. NIV, NEB).
 
The use of the word prototokos in the Bible always means one who has come into existence first in time - before all the rest of his "brothers" - the beginning (arkhe) of his father's creative (or procreative) works. - see pp. 77-88 in Dr. Jason BeDuhn's Truth in Translation, University Press of America, 2003.
 
Yet some trinitarians, however, still insist that the Biblical use of the Greek prototokos can, sometimes, mean "pre-eminent" because they dare not admit the obvious, true, literal meaning of Col. 1:15.
 
For much more, see:

Col. 1:15 "Firstborn of all creation" (Examining the Trinity)
 
'Beginning,' 'Wisdom,' and 'Firstborn' - BWF (Examining the Trinity)

God's "firstborn" (prototokos) Colossians 1:15 (JWitness Forum)

Why doesn't the Sahidic Coptic text see a "genitive of subordination" at Colossians 1:15? (Yahoo Answers)

Col. 1:15 in the NWT (In Defense of The New World Translation)

Wes Williams' Response to Dr. Keay on "Firstborn of all Creation." (Jehovah's Witnesses United)

Wes Williams and Rob Bowman discussion concerning Prwtotokos (Jehovah's Witnesses United)
 
Who Is Jesus Christ? (WBTS)