I spent a couple days in Warsaw and it wasn't bad. Krakow is the city with more to see, though, and naturally more "touristy." Both cities have Old Towns with traditional buildings and such, but Warsaw's is fake. As other posters alluded to, it was all rebuilt, whereas much of Krakow is authentic.
The military museum there was kind of cool, with all kinds of neat stuff like the armor from Polish "Winged Hussars." These were renaissance era cavalrymen who literally had wings affixed to the back of their armor. Don't see that in the Hollywood movies that often, do you?
I did take a day trip out of Warsaw on the train to Malbork. If you like castles, this one's a beaut! It was built by the Teutonic Knights (a German order of crusading knights in the Middle Ages) and called Marienburg, at that time. It is a sprawling, red brick gem on the banks of a mossy, green river.
One tangible piece of advice I can give you if you don't speak Polish: At the train stations, write everything down you want to do -- what date and time train, destination, what time return train or one way and which class service (1st was only pennies more, it seemed). Hand it to the ticket seller with your credit card or cash, and they hand you your ticket back -- no Polish necessary! Worked great for me...Lonely Planet Polish was all I spoke!
Hmm...as you can tell, I'm a bit of a history buff, so all my recommendations lean that direction. Check out my Poland travelogue on my web site at: http://home.earthlink.net/~mikedemana/
Good luck, and have fun!
-- Mike Demana