Wow, this debate has really boiled into something extreme, harhar.
But anyway, let me lay it down like this and I'll do it in numbers.
1. Metaknight is beatable; maybe not all characters can do it, but if you have predicting skill and you know the properties of your opponent, it's highly possible to defeat opposing Metaknights.
2. Yes, Metaknight is broken. This is probably because Masahiro S. put all his admiration into one character that he influenced the balance crew to tip the odds in his favor (but that is all me 1/2 kidding, who knows right?).
3. When you verse one Metaknight, you've versed them all (most of the time, 70% of it). They usually play in one of three manners.
a. One would be the arial beast where they dominate and gimp with fairs, countless Uairs, and sh dairs. Occasionally they like to mix some bairs in with their moves, but that is only to confuse you with their fairs. If this is the Metaknight that you deal with, keep a good distance, shield-grab occasionally, and use projectiles to get them on the ground.
b. The second Metaknight would be classified as the ground combatant. This type will wreck you with close range ftilts and throw you off with A standards and dtilts. The one move they like to abuse the most of any is the dsmash which can finish a good amount of characters off around the 110% mark. If this is the Metaknight you deal with, try short hopping your aerials and use constant shielding and shield-grabbing. If it is to your preference, pivot grab if you like to throw them off, or "try" to dashdance the best you can.
c. The last type of Metaknight is the special-move spammer. 9/10 of the time they will abuse their Mach Tornados and Shuttle Loops. If anything, they will mix this with grabs to knock you up into the air to begin using their specials. If this is the Metaknight you deal with, SHIELD-GRABBING IS A MUST! I can't stress that enough. I know the whole shield grabbing maneuver sounds repetitive, but with Metaknights, it's extremely vital that you apply this technique.
These are just my "opinions", nothing factual or anything like that, just me stating what I feel will help others at least try to achieve better status in the Vs. Metaknight match-ups. Please no flamming, these are just my thoughts on the matter.