From some of the comments I've read by Purp, Dauntless, and Val, my assumptions on Eidolon are seemingly justified, though I haven't really voiced much of them. I've only played 9 matches with Eidolon (hey, don't judge me! I'm coming back from a 6 week hiatus and I've been dealing with some life problems such as dead vehicle, pulled teeth, and the joys [and pitfalls] of prescription painkillers

), but those games and the major theory crafting/debating/gollum-ing I'v done with myself has made me realize a few things about our favorite little bear.
Eidolon of the Great Revel is actually a skill intensive card to play with. I say this because it's quite easy to dig a hole that you can't climb out of with this guy. I did it in my very first match with him, and so did Purp (if anyone bothered to read his States report).
The skill lies in knowing when to ride his ability to a win and when to 'throw him away.' He's an aggressive creature through and through. The opponents know this, and that's why they can make some crazy mistakes, such as throwing mutavaults away to chump him. He changes the entire sequencing of your spells - do you run the bear out and take 2 damage to Searing Blood a creature next turn, thereby putting you 'ahead' in life totals and clearing room for an attack? Or do you burn first and ride the Eidolon out for some potentially additional damage either by attacking or baiting a removal spell that shocks the opponent? Do you try to run 2 out and build your hand up with more burn so you can suicide attack/block and then unload on your opponent?
Position is very important with Pillar Bear. We can't help our position in Game 1 - we either win the dice roll or not. But the fact remains that maindecking him gives us an edge Game 1 against Control, and when we're in a bad matchup with him Game 1 versus,
say, Monsters we are fully prepared with a sideboard to turn that around. I played some games against Mono Black and B/g and I enjoyed the Eidolon while on the play, but I boarded him out while on the draw. My position in each game determined when he'll be at his best. Realizing that lets you use him more effectively.
I know it was mentioned before that YP$ is better in an unknown meta, while Eidolon is better in a known meta. But at the end of the day, choosing to go with the Eidolon maindecked will test your skill of metagaming too. That's big. Looking back on the last two little tournaments I played in (FNM and MNM), I'd say I was correct in playing Pillar Bear at my usual stomping grounds on Friday, but I was an idiot for maindecking him going to a store on Monday that I play at once every 3 or 4 months. Now, will I play the Eidolon tonight? Most likely, since I do predict that most of the same decks and players will still be there and I'll take any little edge that I can get.
I'm pretty sure
that there are some points that eluded me now that I've put most of this out in text for you guys to read, but I think I've delivered the gist of it. On the surface, Eidolon may seem like it's favored by the people that want to interact the least, but I think there's actually a bigger picture to see. I think the more skilled players will be able to be more successful when playing around his "drawback." Just like with Young Pyromancer, I think that playing with Eidolon will force you to make some difficult choices, but the choices you'll have to make are just different ones. The decision trees you'll have to mull over with YP$ are going to be completely different, or even opposite, to what you'll have to decide with the Eidolon.