I absolutely loved the first one, for a few reasons. Entertainment value was great. The story was pretty good too, not the everyman reluctant hero that you see in everything else but an idiot-king. I also loved the production motifs, like when the teacher and schoolkids were stomping on the bugs, and the "would you like to know more?" line... really good at redefining the full reality and not just taping a story onto my existence. The production quality was very bad in some places but was also very good in some places too, and I even used the DVD as reference material when setting up audio-video systems--the space battles had very high video resolution (you can see reflections of faces in the consoles), and the ground campaign had excellent audio (check the scene with the bombing run and crank up the volume). One of my favorite movies ever.
I have never seen any of the others, not even a trailer. I heard they were crap and I didnt want to ruin my attachment to the first one.