I've been to Poland a fair bit and would imagine the host cities; Warsaw, Poznan, Gdansk and Wroclaw will all be great. I heard that Warsaw will have the largest ever fan zone and can imagine the atmosphere will be unreal. Gdansk and Wroclaw are both beautiful cities and are usually buzzing in the summer anyway. And I was in Krakow in 2008 when the last Euros and the atmosphere was great then. They had an outdoor area with a large screen and stands built up, loads of different nationalities enjoying the games together. Plus the centre of Krakow is all geared up for tourists these days.
I can't afford to make it over this year, but if you do go you will have an amazing time and most people are surprised how nice the country is.
Cheers
Was in Krakow last year. Lovely city. Fell in love with the parts of Eastern Europe I visited, particularly Budapest and Krakow.