It added 10 new countries. Overall it looks to have been a success.
Of the ten countries that joined, nine of them have transformed from developing nations to now developed, first world countries allied with western values. All but one — Hungary (still a developing country flirting with Russia).
But why did it expand so much in 2004 —- would that not be so so overwhelming? Even with the labor restrictions, isn’t this massive surge partially what drove the UK to vote for Brexit as they couldn’t handle the massive open immigration from 10 poorer countries?