Sure, there's some foreign investment, but it's going into crap like hotels and shopping centres, not infrastructure that's going to support a decent economy in the long term.
Are you sure? I motorcycled through Poland a few months ago, and I saw some massive spending on infrastructure, especially the A2 motorway. If you've used the roads of Poland, you'll agree that an upgrade is vitally important. In particular, roads leading to Berlin are important for transportation of goods.
Warszawa seemed modern in most regards (probably because it needed rebuilding after World War 2), and they have a very nice subway system under construction (already partly operational) as well as modern surface trams.
Still, most of the population live in thousands of small villages, and those people probably aren't noticing the economic growth too much.
Since joining the EU, costs of production have risen across the Polish economy leaving it less able to compete.
That's what happens when you become a developed nation, no?