Good questions - here's my ridiculously long response
As far as a niche practice, I haven't set one up - I'm 2 years out. I did, however, land a great associate position at one of the highest producing private practices in the state. Also, coincidentally, the owner of the practice is best friends with the CEO of a major 100+ million dental chain. I go to lunch with both frequently (once every week or two) and I've learned more over lunch with them, than dental school taught me in 4 years...or would have taught me in 30 years.
A boutique practice would be a practice that provides an ultimate experience to a certain type of patient. If you were going to buy a new Lexus, would you go to a shady corner car lot, or the fancy car dealership? You would go to the fancy car dealership. Why? Thats your answer. You have the money, obviously, and you want the positive experience. Dentistry is extremely personal and commands a lot of trust. If you can earn the trust of your patients by doing whats right for them and create a practice that makes them excited to go to the dentist, you've already beat the corporations. If you throw in an emphasis on high quality dental work and unique services that other dentists don't provide such as invisalign, botox, occlusion therapy, implants, full mouth rehab, complex oral surgery (impacted thirds, apico, grafting)...you'll have patients that feel unique and special for going to you. Personal phone calls (which are possible because you have a fewer amount of patients to worry about) and maybe a hot lemon towel after the appointment....those type of things. You just have to be willing to work and provide the ultimate experience for your patients...whoever those patients may be. This will attract the segment of society that feels they shouldn't be treated like a head of cattle. It won't attract your run of the mill medicaid patient that wants to cut corners and get away with the cheapest possible treatment. It will also usually allow you to have a lower overhead since your office is smaller, therefore netting you more profit.
A larger group practice (I'm talking private, not corporate), in my opinion, gives tons of flexibility and allows the practice to corner a larger segment of the population because they are capable of working on more patients (more dental chairs set up, more assistant, bigger hygiene department). They can be open 7 days a week. They also have the resources to take on large segments of population care - medicare, medicaid. Some offices will hire associates to treat the medicaid population while the older dentists will take the FFS, private insurance. Others do it differently. Some don't take medicaid at all. It also allows for more staff to be hired since more money is being generated to cover the overhead. Dentists can cover for each other when they don't want to work and keep the office running which is important for money making, emergency appointments, phone calls, a sense of stability in the community, etc.
Any practice size in between can still work, and there are some dentists out there in medium sized practices killing it. I just happen to think the trend will favor smaller, specialized, boutique practices that are dedicated to targeting a certain segment of the population....or large group practices. I think dentists that aren't super creative or willing to work hard will find the profit and overhead of medium sized practices unfavorable over time.
The most important thing I can tell you is to realize this - The only constant in life is change. Those who embrace change will be more successful. If you watch 95% of the news in this country you'll generally hear favorable views of European style health care or European style economics. Over the long term, each election cycle brings us a step closer to socialized medicine or a socialized country. A reasonably intelligent person, despite their political leanings, will realize the direction we are going and they'll find a way to make a buck off it....even if that buck will eventually be taxed at 85%. Dentistry is a great profession and it's a needed skill, which can't be said about many jobs. Back in the golden days you could just open your office and make good money. Now you have to be more creative, the game has changed. The solutions are to accept change and adapt, or change the rules so we can have another golden era.