First I need to point at the fact that the hadith as quoted in Musnad al-Imam Ahmad say that this sermon was in the middle of the days of tashreeq (11-13th dhil hijjah), without saying it was in the so called farewell sermon or any other one. The same information (about the vague date) can be concluded from a narration in sunan ad-Daraqutni. The fact that scholars are using this as an evidence for a sermon on the 2nd day of tashreeq beeing sunnah however seems to strengthen the fact that it was part of the so called farewell sermon.
The hadith as is in musnad Ahamd is mursal, it means the tabi'i abu Nadra أبو نضرة has not quoted the the sahabi on whom's authority he was narrating (abu Nadra says's i've been told by a (the) person whom has witnessed the sermon ...). The rest of the chain is of scholars with a high range of trustworthiness. So the only possible weakness is the fact of this hadith being mursal!
Imam at-Tabarani has quoted a similar hadith in al-Awsat where abu Nadra quoted abu Sa'id (al-Khudri) as the person whom has transmitted to him this narration, but unfortunately the rest of the narrator chain has some big issues.
On the whole the hadith has a sane narrator chain but only the sahabi is not quoted, as sunni scholars consider sahaba as trustworthy this generally shouldn't be considered as a major weakness especially as some other narrations can back it up. But I have not yet found any clear qualification by a scholar.
The meaning of the hadith clearly goes ahead with the meaning of verse (49:13):
... Indeed, the most noble of you in the sight of Allah is the most righteous of you. Indeed, Allah is Knowing and Acquainted.
You may also find it in other ahadith like this one from Jami' at-Tirmdihi, which even quotes the above verse. And this hadith ncompiled in sahih al-Bukhari and Muslim and other sunnah collections, which practically shows that there's no superiority over non-Arabs.