I happened to play Portal 2 before I played Portal 1 (mostly because Steam had 2 on sale at the time).
While playing 2, I didn't really feel that lost. Most of the mechanics shared between the two games (portals, buttons, cubes, moving platforms, turrets) have similar “tutorial” levels. Story-wise, the two main robot characters in 2 will tell you how 1 ended. Which would be a disadvantage if you don't like spoilers.
There were a few references that didn't make sense until after I played 1. Most notably, the appearance of a particular prop in 2's final cutscene.
Having thoroughly enjoyed 2, I went on to play 1. But it was kind of a letdown, having only about a quarter of the play time, and missing the lovable characters of Wheatley and Cave Johnson. I probably would have appreciated 1 more if I hadn't played 2 beforehand.