Disappointing. Shaky plot, mediocre execution, riddled with contradictions and holes in the plot, cheesy conclusion. The supposed big bad aliens are peripheral, the characters are stereotyped and lack substance. It wasn't wholly awful but I find it hard to remember the good parts apart from the chupacabra and the scenes it appeared in.
Considering that the book was written over a 30(?) year period and had 3 authors, it is not surprising that it sucked ass.