Definitely go! I had the great opportunity to work in Warsaw in 1998-99 and it was wonderul. I was able to travel throughout Poland. I agree with everyone's comments that 2-3 days in Krakow is sufficient. Definitely go to Auschwitz and the Salt Mine's. I hired a taxi to take me to Auschwitz (sorry, I don't recall the price). I took the city bus to the Salt Mine, it's incredible.
In Warsaw, there is plenty to do and see. There is a good market across the river from old town in an old stadium (your hotel staff should be able to assist you) the tram will drop you off right there. But use CAUTION!!! There are good deals, but along with that there are plenty of pickpockets too. There are some great crystal and ceramic shops in WAW. The beer is wonderful, as is the chocolate (Wedel - yummy!). I always took the train to Gdansk or rented a car and it was from WAW. If you fly, you won't get to see the countryside, but will definitely get there much faster. Good luck and enjoy!