timppu: So how did you do it? If with VPN, do you have to use it also when playing the game?
I asked a German friend to log into my account and activate the game. Heck, I'm a German citizen anyway, if he hadn't done it I would have done it next time I go there myself. :P
There don't seem to be any problems after a successful activation, I also added DLC bought through the Polish Steam store without any issues. And I move a lot between Germany and Poland and haven't ever had any issues installing and playing games that I bought in "the other" country (although reportedly Sleeping Dogs was censored for German users who had used a UK key retroactively - so I'm a little afraid that one day Steam might do that to some of my games if I log into my account in Germany).
timppu: but is it possible to run into similar cases there?
I'm not sure whether you mean the part with lying about the address or finding games cheaper in other European countries. One thing I can say is that I did not have to lie about my address or anything, it was a legitimate order with my real IP and billing address and Dark Siders 2 was the only case so far where Steam didn't allow me to activate a UK serial in Poland. As for the offers themselves - I've already found many games on Amazon.co.uk much cheaper than on German Amazon, some even cheaper than in Polish retail (which totally blew my mind). For Dark Souls I paid approximately one third of the German price, curiously even ordering a release tagged as "UK import" in German Amazon would have been almost twice as expensive as directly from Amazon UK. Also the delivery is free throughout the whole European Union if the total value of your order surpasses a certain value (20 or 25 pounds, I think) and while it says 7-10 business days it only took two business days in case of my orders. So considering that I don't even have to lie about my location and get a free delivery ordering from Amazon UK seems to be a good idea.