5 of my current bettas in 1gals have lived for ... 2.5 years now ... with no fin biting going on and they're very obviously happy. No, I don't change their water every day or week or month, either.
Zombiekeeper, you're really not being very polite, just because someone disagreed with you. And personally, I feel having bettas in 1-gallon tanks whose water isn't changed even every month--as you state above--is flat-out cruel.
It never ceases to amaze me how upset people get with each other over these things. Honestly, I think whatever your fish is happiest and healthiest in and is easiest for you to maintain is best. No need to get huffy as long as the fish are well cared for and loved!
I have experience with multiple tank sizes and personally I much prefer larger, cycled tanks. I had my male halfmoon in a 1 gallon for several months and he was miserable. The combination of small space and hard water (not much I can do about that part) resulted in his anal fin rolling up on the bottom. His fins are so huge that he had a hard time moving around in such a small space. Ever since I bumped him and my veiltail up to a divided 20 gallon they've both been far more active and healthy - no more tail biting and curling fins! They absolutely love the space.
Plus for me keeping up with water changes on the 1 and 2.5 gallons was exhausting - I'm a full time college student and it was hard to find time sometimes.
But seriously. If 1 gallons work for you and your fish, go for it! The only time I will ever tell someone that their fish habitat is not okay is if the 'tank' is less than a gallon and can pretty much fit in the palm of my hand (my friend once insisted on keeping hers in a cup) or is filthy, or the fish are clearly miserable.
All my boys are in 3 gallons, and my girls are in 2 gallon tanks. I just upgraded Chappy from a 1.5 to a 2, and Ray Charles from a 2 to a 3 after his brush with popeye, and I honestly think that it made them both happier :D
I don't see upgrading the boys anymore, but sooner or later, my girls are getting their own 3 gallon homes! And yeah, sometimes it can be a drag to do those water changes, but I wouldn't have my dear fishies if I couldn't handle the responsibility.
Yeah, I'm one of those people who think 2.5g should be the minimum, but someone with time and experience could keep a betta in a gallon...and a half (lol). I don't know, I think larger tanks keep them more entertained, too.
In my opinion, it all depends on the fish. Merlin was actually very happy in his one gallon, but that was also because he'd never known anything larger. Now that he's in a split 5.5 gallon, he wouldn't go back to a one gallon. Dragoon on the other hand, became a bit more active in a one gallon while he was in quarantine, but as soon as he got a larger space he really perked up. As several people have said it really depends on the betta and how much effort the owner is willing to put in. I couldn't handle the upkeep of a one gallon, which is why I have the 5.5 gallon.