My thoughts :
This novel is clearly sci-fi : first contact with aliens, can you get something more obviously sci-fi? And yet, it is more an exploration (and a critique) of the US justice system. The outsider points of view helps, because the lawyer has to explain everything to his alien client, Hask. It shows the unfair aspects of the justice system, even if, in the end, Sawyer's message is not entirely pessimistic : we finally learn the truth about Clete's death. I really liked this aspect of the book : the idea that there is more than meets the eye in a particular situation, that you have to ask questions and look for the truth (even if lawyers and the DA are not concerned by the truth, more by strategies, jury choices, ...).
The end of the novel is honestly not the best : I felt like the author used an alien ex machina to tie his plot, but then, the real point of the plot is the trial, not the sci-fi aspect, so I can forgive it.
This novel also introduces an important theme, even if it's not original : religion, science and how people can live with those two "forces". More specifically, the theory of evolution has a large part in the later developments of the plot, and Sawyer clearly take a side : he shows how ridiculous it is for people to believe religious arguments/ideas when there are facts that contradict them. For the record, I totally agree with Sawyer : anyone who argues against the theory of evolution because it's not what the Bible teaches us need a serious reality check.
Verdict?
Illegal Alien is a good novel, but don't read it thinking you'll find advetures and traditionnal sci-fi themes. It explores US society and points out what could be improved in the Justice system. I found the trial engrossing, and the three main characters (the lawyer Rice, Frank Nobilio who is the White House representant among the aliens, and Hask) quite well developped. The other characters were fonctions in the storyline. I think this book is a good read for somone who is looking for an unusual sci-fi novel, or for someone who is discovering the genre but doesn't want to read a space-opera or a post-apocaliptic dystopia.