Let's be honest here. You're probably interested in this series because of the movie's reputation, and you're wondering if the TV show is worth watching. Let's get this out of the way first: you CAN skip the TV series and just watch A Contact (the compilation movie) followed by Be Invoked (the new and improved ending). This may be fine for you: you'll get skip some of the repetitive bits of the series, but you'll also skip characterization. While the TV show is a bit slow at times, A Contact and Be Invoked try to cram everything together... things develop abit too fast.

The problem with Ideon becomes apparent as soon as you watch the first episode. At its core, the show is a series of 22-minute commercials for toys which haven't been sold in decades for a country on the other side of the world. Animations are reused heavily, early plots mostly follow a predictable formula (land on new planet, aliens attack with new strategy, mech pilots save the day), the characters can be hard to like, and you get tired of the revolving door of villains. But there are also episodes where the show takes itself seriously. Tensions between the main characters are handled with respect, and most of the main cast develops quite wonderfully, even if it seems a bit slow. The plot is sometimes stitched together with crude seams, but at other times it flows quite well.

Once you get to episode #23, the tone changes, and the series discovers its purpose. Remember how the conflict in episode #1 seemed almost accidental in origin? By episode #23, the conflict has grown so large that no matter what anyone says or does, the fighting will continue, and the consequences are always serious. Even if you save the day, you can't save everyone. Even if you surrender, you can't stop fighting. Even as you work towards the same goals, you can hate and betray each other. The plot picks up the pace, characters develop and break down quite spectacularly (they don't cope with things by whining about them, thank goodness), and even the deus ex machina that saved the day in the first half of the series gets turned on its head. Everyone gets a moment before the end to show their true depth.

Episode #39 should be skipped, just watch "Be Invoked" instead, since episode #39 was recut, improved, and made into the beginning of the movie. It is a great episode, but the movie gives you a better version.

All said, I would give a 5/10 or 6/10 to the first 22 episodes, with the acknowledgement that a few of those episodes are fairly good. I'd give the last 17 episodes a 8/10 or 9/10—again with the acknowledgement that there are a couple stinkers, such as episode #34. I might rewatch this someday, but I'd skip over most of the episodes.