I think it has a pretty decent chance of whiffing, cast on any turn other than turn one. Most Riptide cubes have pretty low curves, and not a lot of spells with mana costs above three, which might make this dicier than in your traditional cube.
Assume that 30% of your cube has a cmc of 4 or greater (my own is closer to 27%, but I'll round up). Then further assume that your opponent's deck's curve also follows this ratio - that is, of their 23 non-land spells, 30% cost 4 or more. Then, using a hypergeometric calculator, and some ugly ass tables:
Population size (# of cards in deck): 40
Number of successes (30% x 23): ~7
Number of successes in sample: 1
Sample Size ___ P(X >= 1)
____ 7 __________ 77%
____ 6 __________ 71%
____ 5 __________64%
____ 4 __________55%
____ 3 __________45%
____ 2 __________32%
____ 1 __________18%
Even on turn one, you have roughly a 23% chance of getting a zero-for-one against a random opponent, which doesn't strike me as a card being worthy of a slot.
Maybe it has merit as a purely sideboard card, against the big ramp and control decks, though.