I don't think that ANY straight that uses a half DE blade can be any worse than the blade you stick in it. Personally, I'd buy the cheapest one because you'll never use it again after you buy a real straight. I bought a cheap Paki one and it worked just fine (after I put a good Gillette blade in it, that is). I imagine it would be OK to learn on. I learned with a real straight, I bought the disposable one out of curiosity (it was $1.00)
Pawel, I see you're in Poland. I have a bottle of Polish potato Vodka in my freezer at this very minute. I must say, it makes a great martini. It's also very good on it's own.