Should I play Portal before Portal 2?

I think it's definitely worthwhile playing Portal first. I just started Portal 2 and would probably have been a bit lost without having played Portal first. There is almost no introduction to how the portal-gun and other stuff works in Portal 2 and there are a lot of references to the previous game.

It makes Portal 2 an even better game if you have played Portal.

Portal 2 starts out fairly simply thus, from a gameplay standpoint, you won't have a hard time figuring out the mechanics of the game.

However, Portal 2 continues the storyline from the first game. There are also many elements in the game that you wouldn't understand/enjoy/love not having played the first game.

Playing Portal 2 without playing Portal can be likened to hearing the punchline, but not the joke.

Furthermore, if you EVER wish to play the first one, it would be ruined by having played the second one first.

Certainly a lot of the story elements will make more sense if you have played the first game, however I don't think it's absolutely necessary.

As for gameplay instructions and explaining how things work, yeah, you'd be much better off playing the first one first. Portal 2 drops you much more suddenly into the thick of things than Portal 1 did.

Those reasons aside, you should get Portal 1 simply because it's a kickass game!