Social media has played a powerful role in driving engagement as well, offering a way for these fitness studios to build a community online.
Australia-based F45 training, which has about 800 studios around the world, focuses almost exclusively on online marketing for future growth.
"I don't think people pay a lot of attention to billboards and TV and radio and things like that anymore as they do to social media — Facebook, Instagram — so that's been our biggest tool for getting people in the door," said Luke Catenacci, co-owner of F45 Training Flatiron.
"Social media is an important tool for us as far as building awareness," said Kari Saitowitz, founder of Fhitting Room. "It's such an easy way for our current clients who are ambassadors of our brand to share their experience with others."
But skeptics question whether the strong demand for these boutique classes can continue, arguing that the barrier to entry is relatively low plus millennials have a reputation for being fickle.
However, for now it seems to be working.
"Our retention rate for paying customers is extremely high — probably around 75-80 percent," said Catenacci.
Saitowitz and Neiman said retention rates are high at their respective studios as well.
But competitors are popping up to meet this thirst for innovative workout regimes. Several millennials CNBC spoke to said Barry's Bootcamp, Tone House, Peloton Interactive and FlyWheel, among others, are frequented. Rowing (yes, that's right, like row your boat) has also been getting some buzz, with CityRowe in Union Square.
Fitness pros say sustainability, scale and building a niche brand will be the key challenges for these studios. These long-term risks are not just relevant to new entrants but even companies such as indoor cycling veteran SoulCycle, which was founded in 2006 and is still waiting to go public.
So, how would I, as the target audience of all these offerings, rate my experiences?
While the high price is annoying, the lively energetic atmosphere in these group classes is a great motivator, and makes me want to keep coming back — something many of us have struggled with at traditional gyms.