This review is for the Terminal 1 US Transborder lounge. There are multiple Plaza Premium lounges. For example, the International one I went to awhile back in Toronto was much better. The star rating is for its class; in other words, comparing this lounge to other airline lounges and non-lounge experience.
Summary:
It's decent if free, not worth it if you're paying for each visit.
Advantages (compared to other lounges):
1) There's food. The US airline lounges tend not to have food. Air Canada Maple Leaf does, so does Plaza Premium.
2) There are Plaza Premium lounges in multiple terminals and sections (domestic/transborder/international) so a membership might be useful
3) Soup is available even in morning. For AC lounges it's only available after X time.
4) Showers were available but I didn't use.
Disadvantages:
1) Juice machine wasn't working
2) Breakfast selection was limited
3) Table seating was limited. It was mostly lounge chairs. Good for sleeping I guess
4) Washrooms weren't the cleanest... definitely not bad tho!
Comparison to AC Domestic
Plaza Premium US Transborder consisted of cereals, baked goods, poached eggs and beans.
AC domestic lounge with eggs, bacon, sausage, working juice machine, etc.