How valuable/impressive is a review article written by an undergraduate?
There are several factors to consider here when evaluating the "significance" of a review article:


The length and scope of the review article: a six-page review of recent literature with 50 references is not as impressive as a 50-page review with hundreds of references.
The publication in which the article appears: is the journal a high-level, review-only journal, such as Reviews of Modern Physics or  Chemical Reviews, or is it a review article in a traditional journal?
Was the review invited or contributed?
Is it part of a special issue? (If so, the standards for acceptance are often a bit laxer than in a "regular" issue.)
Since it's written as an undergraduate, you might try to compare its citation count to similar review articles written around the same time as a proxy for its significance to ongoing research in the field.