I think most of you are missing the point of the film here, its a social commentary, the aliens were an allegory for corporate greed and capitialism at the expense of personal freedoms and human rights. It was made in the 80's, Reagan era, when these things were being pushed aside in favour of power and a money making screw everyone else culture.
Personally, I think it is brilliantly done, Piper is a pretty good actor (for a ex wrestler) and the tongue in cheek script and direction makes sure its doesnt get too caught up in its own message. Although, having said that, the message is pretty loud and clear by the time the two heroes get into the alien base and find out humans are collaborating with them, just for a guaranteed increase in their investments.
Keith David is great also, the pointlessly long and over the top fight scene between the two is one of the best thing about the film, just a bit of fun interjected into a serious film. The scene now has cult status, and was immortalized by Southpark, in the episode `cripple fight' where Jimmy tries to get Timmy to put on a silly hat. Brilliant stuff.
The film could have done with a bigger budget, but I still rate it as a classic, with a message that is perhaps even more relevant today; what with the political propaganda machine even more powerful and insidious than ever, and people even more stupid, it seems.