Poland was doing just fine in the years before accession.
20.7% unemployment in February 2003, before EU accession.
Economy grew only 20% between 1989 and 1999.
Considerable devaluation in the currency at the first half of the 1990's, with several unplanned devaluations due to the economic situation.
Deficit of 7.6% of GDP in 1999
There's more... Poland wasn't doing fine at all, and with huge problems such as the Szczecin Shipyard (which was a great example of financial manipulation) falling - eurofound.europa.eu/publications/article/2002/bankruptcy-of-the-szczecin-shipyard - the early 2000's were pretty horrible for Poland.
Serbia and the balkans arent too bad off either.
They're doing pretty badly. Serbia's GDP in 1990 was around 14,000USD and it's just crossed that line in the last few months again. Kosovo is a basketcase, Bosnia is going nowhere (GDP was somewhere between 7-8bn USD in 1989, these days it's 5bn), Albania has 33% of the population living below the poverty line... should I carry on?
Look around Lower Silesia and Wielkopolska. The place is full of factories and warehouses that cater for the German market. Without the EU and customs barriers in place, what happens to those businesses?