As much as I love The Binding of Isaac, I can see why certain people might be hesitant to blindly buy, even with the affordable price. You now have one less excuse, however: the game's creators have put a Flash demo up on (where else?) Newgrounds.
Featuring two levels, eight bosses, and 60+ items, the demo is a good glimpse at how the full game handles and should tell you whether or not this is your jam. According to Edmund McMillen, sales of Isaac are currently rivaling Super Meat Boy's release numbers.
As for what's next, expect a free Halloween update that introduces 20+ new items, three new bosses, another playable character, more achievements, and a "final chapter," which "should be a much bigger challenge." Until then, more bug fixes should continue making the rounds.