Been a number of times, but mostly on my own. Last went with my wife about 13 years ago.
Beautiful city, but I recommend going in the warmer months (June was ideal, but still can be on the hot side). Auschwitz and Wieliczka are both worth visiting. You've also Schindler's factory, which is now a museum. Kazimierz, the Jewish quarter, is lovely to walk around.
There are places where you do have to be careful in regards to nightlife, but plenty of bars and restaurants. Prices in Poland have definitely gone up over the last few years, but still cheaper than here.