I read many of the negative reviews, but ended up giving this a chance based on the cast. The central theme of this sci-fi suspense thriller focusses on the question, "How do you know if you are really you?" Imposter has enough visual effects to make you believe its the year 2075 (in Johnny Mnemonicesque fashion), yet focusses on the characters. The cast was great, with every actor/actress living up to their reputation. Sinise and D'Onofrio are especially terrific.
After an initial intense beginning, Imposter eases into a nice fugitive-like flow. This movie has most of the things we like in our Sci-Fi: aliens, fight scences (guns and a little kung fu), futuristic chase scenes, neat mind-twisting philosophical mumbo jumbo, a terrific ending and a little CGI thrown in for good measure.
If I had one complaint its would be the lack of gratuitous T&A. If this really was a "B" movie as many of the reviewers suggest, the T&A would be there (as I had hoped when I purchased this). As for me, I liked it better than the other Philip K. Dick-based movies, Minority Report and Total Recall (and I love Arnold). Imposter does not come across as a "B" movie. While I would not put this in the same class as say Blade Runner, few movies are.
Bottom line - Buy it, you'll like it.