Link's and Toon Link's recovery (not the tether) is easily counterable, as are Fox's, Falco's, Wolf's (not their illusion moves though), Bowzer, Ness, Lucas, Donkey Kong, Luigi, Mario, Charizard, Samus (not tether), and Wario. You can counter Diddy Kong's, but it's harder to predict. You can also counter Marth's, but due to its speed, it's kinda hard.
I've been using Counter as an edge guard for months, and I find it pretty useful, though others on the Ike forum will say its a waste, and spiking is a better idea. However, I find it easier to counter their recovery, and with a higher success rate (for me). Most of the characters you can counter can't make it back, assuming they are at a decent % (which they probably would be, if they're using their third jump to recover to the ledge).