Rory Manion of 1Up.com gives Twilight of the Arnor a glowing grade, rating it an A on its review scale. 

Torn between the desire to have a peaceful, altruistic empire and the need to wage a four-front war, this review covers the wide range of new possibilities present in Twilight of the Arnor quite well.  Rory's regime was at times kind and giving, and at others brutal and unrelenting, often destroying entire stars, and enacting terrible genocide programs.   Overall, he sums his thoughts up on the expansion with this:

"For better or worse, Arnor is an add-on that completes its parent game rather than simply expanding its scope. Some of the minor adjustments, primarily those made to the interface, seem better suited to a patch than an expansion; others are so immediately and obviously integral to enjoying the game that -- and I hate to abuse this expansion pack cliché -- I honestly can't imagine reverting to a previous version."

Read the full review here.