The honest answer on whether it's worth it
Niagara Falls is approximately 400 miles from New York City. A genuine day trip — leaving early, spending a few hours at the falls, returning the same night — involves around 10–12 hours of travel for 3–4 hours at the destination. That's a lot of travel. Whether it's worth it depends entirely on how much you want to see the falls and how limited your time is. An overnight trip to Niagara makes significantly more sense than a day trip.
By train
Amtrak's Empire Service runs from New York Penn Station to Niagara Falls, NY — about 8 hours each way. Too long for a day trip. The train is the right choice for an overnight trip: depart evening, arrive morning, see the falls, return the next day.
By bus (most common day-trip option)
Several operators run coach tours from New York to Niagara Falls departing very early (typically 6–7am) and returning late evening, total journey around 7–8 hours each way, giving 2–3 hours at the falls. This is the "day trip" most tour operators are selling. The bus ride is genuinely long; bring something to read or watch.
By organised tour
Multiple tour operators run guided day trips from Midtown Manhattan. They handle all logistics, typically include Maid of the Mist boat ride tickets, and provide commentary en route. More expensive than the bus but more seamless. Search specifically for "Niagara Falls day trip from NYC" to find current operators and pricing — the landscape changes and we don't list specific commercial operators here.
What you'll actually see
The American Falls and Bridal Veil Falls on the US side; the more dramatic Horseshoe Falls (Canadian Falls) just across the border in Ontario. The Maid of the Mist boat ride goes directly into the mist of Horseshoe Falls and is the best way to experience the scale. Cave of the Winds walk is also excellent. Both require tickets purchased in advance during summer.