I love this card. It enables an obvious and, frankly, unused deck type, one that incorporates a completely unique experience into magic: the gathering: the coin flip. Any coin. Could have Chucky the Rat, or Queen Elizabeth, it doesn't matter. It's this strangely kinematic thing introduced over an otherwise stationary play area (except for tapping). It's such a fuck-you to hardcore magic players that if ever a competetive deck is made of it, I want to be there.
This card takes a 0.5 probability for a coin flip and, with no backup, turns it into a 0.75 probability. This can make some cards that have coin flip "self-destructs" or other negative effects as drawbacks downright great. But consider the effect of a Mirror Gallery, allowing two Krark's thumbs in play at the same time... I believe this gives you an infinite number of potential chances at your choice flip, or a probability of success approaching 1, as the flips reference each other ad infinitum. There are several cards that can win the game if played with a guaranteed coin flip win (Game of Chaos, for one). But better, I think, is the advantage afforded a dedicated deck with the increase of 0.5 to 0.75 probability per flip. This is a fun mechanic (coin-flipping)! There is no way that Wizards will leave this mechanic untouched forever. There will be more coin flip cards.
Also, I have not seen anybody talking about this in print, though surely someone has: When are we getting the next Unglued, Unhinged, Un-set?! We are overdue. I'm sure it's a 'rainy day' project at Wizards, but holy crap they would just sop up the money with that. Krark's thumb would surely benefit from that. However, I think coin-flipping will one day be splashed back into Standard. This being said, I could be completely off-base with my estimation of people's desire to flip coins when playing magic.
Stitch in Time and Fiery Gambit (both also good investments right now) become really strong with the unaided presence of Krark's Thumb. There are so precious few coin-flip cards now. There will be more. An Eternal deck will one day be viable, if it's not already. From the looks of the prices of the likely key cards, there are decks brewing out there.
Get in low for a long-term hold. Go FOIL for investment purposes. They could always throw this into a Commander set, if a Commander that utilizes coin-flipping ever emerges. Plus, FOILs seem poised at this moment to gain hugely (potentially from $3 to $20) within the next year or two, whereas non-foils are more likely to continue to slowly gain for years, unless a viable deck emerges in some format (and in that case, FOILs will explode, too, so just go with FOILs).