I started with this method, and in a few weeks I had people coming up and asking what the hell happened to my voice. Never practiced before, just sang.
Now, many methods later, I can relate to those saying that it is not the method, but the systematically hours put into it that counts.
That said: It's also important to be ware of what you want with your singing. For me, being a cover artist, copying other artists was very useful in creating the different techniques I needed to be able to deliver variety in my show. Cover artists that sing in a very limited range with one style and one sound/tessitura/voice tends to be very boring to listen to. AC/DC and Johnny Cash It is supposed to sound different.
I would also recommend copying other singers if you want to have a few techniues to choose from. Try saying that a guitar player don't learn other guitarists techniques, it would be insane and don't make sense!
Yeah, after a few weeks it REALLY changed my voice, too. That's why I stuck with it. A year later, I still feel changes inside my body (especially increased air flow and resonance from my lower stomach all the way up to my throat).
A lot of people recommend vocal workouts. Other people, say don't do vocal workouts and just sing instead. Singing alone never really helped me, but vocal workouts alone isn't enough either.
So, if possible, do both. You learn different things from each one. Vocal workouts exercise your voice (think of it as a physical workout to get better pipes). Singing songs help you to learn the art of singing tracks.
I tried singing songs every day, but it never seem to improve my voice at all. So I decided that singing songs wasn't the best thing for building my voice. I turned to vocal workouts and instantly felt big changes within 2 weeks.
Now I've done vocal workouts for over a year. So my voice feels much more agile, flexible, and hitting the right pitch is almost effortless now. It's an incredible feeling. But I never really practiced singing songs seriously. So I have trouble controlling my voice in the context of a song. So that's my next area of focus.
Also, if you can afford it, get voice lessons. Find a teacher that you admire and is able to honestly critique and shape your voice in a way that you like.