How did this one get under the radar? Week after week, we get dimwitted comedies, and then a charmer like this comes along, and it gets a limited release. "Why Stop Now" is a bright screwball comedy about one fraught day in the life of a piano prodigy, his crackhead mother and her drug dealers. It's the first movie in memory that when the kid is performing onstage and the door at the back of the auditorium opens, he does NOT want a parent to appear. "Why Stop Now" opens with the crucial audition of Eli Smith (Jesse Eisenberg), who could win a scholarship to a famous music conservatory. He arrives late and explains to his teacher that he has had a very hard day. It began with his driving his mother Penny (Melissa Leo) to a drug rehabilitation clinic. She isn't enthusiastic about this; he nearly has to carry her in.