I did the horrible stupid thing that you absolutely never should do and read Alternative first, and you know what? It is still an excellent experience.
There are like two things that are confusing in the first minutes, then it catches you up to speed and everything is awesome. I do not understand why people who calls Extra shit (it's actually pretty good in my opinion, but i like that kind of thing) keep on insisting that you have to read it first, sure there's some character development and you might be a bit more connected to the characters at the start of Alternative but at the end of the day Alternative is certainly capable of standing on it's own two legs.
I would say give Extra and Unlimited (that's the boring part in my opinion) a try, if you don't like, fuck it, read Alternative.