It does what it claims to do quite well. I like the fact it is international in scope because I travel for work a lot.
Unfortunately the very first holiday I checked was wrong! In Canada, in 2010, Victoria Day is May 24 not May 17 as the calendar says - so it loses a star. Hopefully the developers will notice this review and fix the error.
(5/10 update - restoring the 5th star because the Victoria Day bug I reported was fixed promptly!)
Dave Ings about Holidays Calendar