What kinda gets me is the name Łukasz, because in German and English it is indeed an L and all the ones I know actually use the L (maybe unless talking to other native speakers).
Is it still common? It was popular when there were many encodings that always broke and when people wrote SMS by pressing number keys 5 times for 1 letter, now I don't see it that often. I'd say it's about as common as writting l8r in English :)
By the way, never try to use Polish letters in a Windows username. It works, but every once in a while something third-party or rarely used breaks unexpectedly.