I am from Warsaw so... yeah, I have been there. A few times

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In general, I recommend Kraków to those visiting Poland for the first time. It is a beautiful city with a very pretty Old Town district. Warsaw is by far not as pretty. If that does not bother you too much, Warsaw can be an excellent destination though. It is at the center of so much that happened in recent history: It was completely flattened in WW II, parts were rebuilt brick-by-brick (visit the Old Town, you could never tell it was all rebuilt after Mr Hitler decided he wanted to see it all in ruins), other parts are impressive examples of Communist architecture - most of all the Palace of Culture and Science in the very city center. In the last dozen years or so, Warsaw has seen an incredible construction boom (now halted by the recession of course). Before the boom started slowing down, Warsaw was on its way to becoming the city with the most skyscrapers in Europe. Especially compared to low-rise Dublin, Warsaw is quite a sight. You get a fascinating contrast of crumbling 70ies concrete and shiny glass and steel.
The shopping landscape is just as full of contrasts as the buildings. There are shopping malls everywhere, offering just about any brand under the sun. But there are also markets (rather well hidden, so do your homework if you want to visit them) selling all sorts of cheap junk

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If you want to see some of the prettier bits of Warsaw, head to the Old Town, New Town (just north of the Old Town, walk through the barbican) and Wilanów palace (bus 519 from Emilli Plater or taxi).
If you want to see history, wander around the central district, pay attention to the mad contrast as you turn a random corner, make sure to visit the ghetto area, the Monument to Victims of Deaths and Murders, the Palace of Culture and Science, the Warsaw Uprising Museum and to view the film about how Warsaw was flattened at Hitler's whim that is shown in the Old Town square. When visiting the Palace of Culture and Science, you can take the elevator all the way to the top and enjoy a nice view of town. Or you can visit the Science Museum - I love this one as they ran out of funds some time in the 70ies and it portrays "science", "technology" and especially the country's future as a center of heavy industry as the communists saw it back then - it is a trip to an eerie past...
Edit: I forgot to mention the Royal Route, extending south of the Old Town. This has recently been refurbished and is now a mostly pedestrianized street well worth seeing. Many beautiful buildings lie along it, including the Presidential Palace and Warsaw University.