I think the reason this innocent Ooze got overlooked is that he's not particularly exciting on the surface. He's only a 1/1. However, he is indestructible. The only way to permanently deal with him is to exile him or force him to be sacrificed. Also, as soon as he swings that he immediately gains a +1/+1 counter. All he has to do is declare an attack. On top of that, when a creature dealt damage by him dies, the Ooze gets yet ANOTHER +1/+1 counter.
Back in May 2012, I said that Predator Ooze was a sleeper card. I saw him becoming very relevant when Scars of Mirrodin block rotated out of Standard. I suggested to pick up your copies on the cheap before he became a mono-green menace. This was excellent advice. His price nearly doubled when Return to Ravnica debuted and doubled again shortly before the release of Gatecrash. This was due to Gatecrash's awesome Evolve mechanic that interplayed beautifully with Predator Ooze's counters. It topped out as a $5 card for a bit.
Unfortunately, Predator Ooze never found a permanent home in any dominant deck in Standard. Mono Green aggro was a deck and he found his way into some lists. But he mostly acted as a way to counter other aggressive decks out of the sideboard. Unfortunately, be didn't get to enjoy a standard environment with Theros devotion in play, as Dark Ascension bowed out of Standard on that set's release.
The good news is that despite not living up to the pre-Gatecrash hype, people continued to brew with him in Modern. Some early versions of the Tooth and Nail decks in Modern utilized a copy or two of him. Some Mono-Green Devotion lists would try him here and there too. Really, though, the best home he found was in Commander where +1/+1 counters are extremely abusable.
It seems like in most sets now Wizards is adding +1/+1 counter support. The enchantment Hardened Scales would seem to be his best friend, giving him an additional counter every time his abilities trigger. Unfortunately for him, Managorger Hydra got printed and took that three-drop slot in the deck. Despite being awesome for devotion, the Ooze got outclassed by the Managorger's ability to not only grow faster but also have trample as well. So Commander is the only real home that he's found. All of this has kept his price tag above $2 however. He's no bulk rare.
What's held him back in Modern is not only the prevalence of Path to Exile, but also the presence of Dismember who doesn't care about indestructible as long as the ooze is 5/5 or less. Liliana of the Veil's sacrifice ability is also relevant but in the type of deck that Ooze is played in, it's doubtful that he's your only creature on the board. Still, when two of the premier removal options in the format can deal with him as easily as almost anything else, you're going to go with the creature that can get a lot bigger, which is the Managorger Hydra.
Going forward, Predator Ooze is probably only a Commander card. It's hard to say if another deck in Modern will actually ever need him, although he could figure into a Hardened Scales strategy at some point. Down the line he'll probably get reprinted and his value will shrink substantially, but until then, this is a card that still has some value.