Warsaw is mega, cheap drinks, fit polish girls and a good laugh. Also the indoor market stalls have some weird shit as well, I was really close to buying a massive fur jacket for like £40 worth of zloty. I also recommend Wroclaw and krakow if you're sticking about in Poland. I talked to a polish hobo who had a Middlesbrough shirt on for about an hour In krakow and bought him a McDonald's, top lad he was.
As others have said, prague is great and I can back that up, however it's more expensive. But tbh it's still really cheap compared to a night out in Manchester, and about a million times better. If anyone's planning on going to prague though, watch out for the weird underground bars, they are dodgy as fuck. Went into one and there was a dead dog on the floor, the walls covered in graffiti, a man with a knife in his hand and loads of people doing a different drugs, just starting at me and my mates, we had one drink and fucked off before we got killed.
All in Eastern Europe is fucking class, reccomended.