Warsaw is a lively capital - a great centre for entertainment, with broadly friendly locals who are always keen to try out their English, and a large ex-pat community. Aside from Krakow this is probably the only place in Poland you will get away with speaking no Polish whatsoever. Spend a week here if you are in a drinking or clubbing mood - prices aren't rock-bottom, but they still beat western Europe, but you can have every drink in a different bar or club for a fortnight and never run short.
You might run short of fun for the daytime however. Warsaw was completely flattened by the Germans at the end of the second world war. Really flattened. There are only a few hundred buildings in the entire city which survived. Suprisingly there is still an old town, complete with marketplace and royal palace, but along with Nowy Swiat (New World Street), and a couple of other streets, these were painstakingly rebuilt from scratch in the post-war period. This is of course worth seeing just to marvel at how well it has been done. Other buildings of note include the enormous Palac Kultury (Palace of Culture), which looms over the city from a massive square near the station. Although it is clearly one of Warsaw's few assets, it was a "gift" from Russia and is a much-hated symbol of Soviet repression to a large proportion of the population. It is always in some danger of being demolished or hidden by new building. There really isn't much else to see, so if you are not keen on nightlife you will probably want to get out after a couple of days.