The problem of Berlin is that there's too much available 'ground space' for the city's 3.5 million population and not enough offices and industry in the area.
Even if more and more multinationals move their German (or even European) headquarters to Berlin, the city does not have the money input of richer cities as Frankfurt, Munchen or Dusseldorf. Rental income in Berlin is not very good due to oversupply and capital growth is very mediocre as well.
If you have the luck to invest in a 'hip & upcoming' neighbourhood, maybe you will make some money but for me Berlin remains one of my favourite hang-out spots, but I would not invest there for the moment (the market is flat - so there's no hurry anyway).
Krakow could be a good bet, with it's population of office workers, students plus tourists. All the numbers I see for Krakow are very good and promising; the only reflection I have is that 2-3 years ago would have been an even better time to have bought there - prices have increased already 50-70% (depending on the source) over the last 3 years.
Concerning Budapest, if you do a little search on this forum you'll find some very good advice. In short: don't expect to make some quick money, but for a long term investor Budapest can be interesting. On this moment, there are a lot of crap projects in the market (bad location, construction & finishings) so be extra careful if you see wonderful 3D renderings of buildings in 'upcoming area'. Chances are big that they try to sell you a capi-block in a dangerous hood.
I don't know what is your first investment in BP, but you seem happy and confident so I hope you made a good choice.
If you look for a good investment, I would also have a good look at US business cities as Chicago or Atlanta. With the current credit crisis, there are some golden opportunities!