The good thing about romaji is that it lets you get your foot in the door right away. You can start reading some words and doing some studying immediately.
The bad thing about romaji is that Japanese doesn’t actually use an alphabet, and it will mess you up. My opinion, from the highly expert position of being a person who has been studying Japanese for all of 3 and a half months, is that you should abandon romaji as quickly as you can.
Let me give you an example.
Japanese adjectives come in 2 flavors: na-adjectives and i-adjectives. Or, to be more precise なー形容詞 and いー形容詞, and there actually is a good reason for me to be more precise. Here’s what I mean:
Read a book that uses a lot of romaji and you will be told that i-adjectives end in i, which, of course, THEY DON’T BECAUSE JAPANESE DOESN’T USE AN ALPHABET!!! Japanese uses a syllabary.
Okay, okay, I’m getting picky here, but there’s a reason for it, I promise. Hang with me.
Let’s look at some adjectives in romaji: genki (healthy), suki (likeable), onaji (similar). Obviously, these all end in “i” and are, therefore, obviously, i-adjectives.
Except that they aren’t. I have to stop thinking like an English speaker and think like the Japanese speaker I hope someday to be. None of these adjectives end in い, so they cannot be いー形容詞.
Let’s move to hiragana.
Genki is げんき, so it ends in き not い
Suki is すき, so it ends in き not い
Onaji is おなじ, so it ends in じ not い
In romaji these all look like they might be i-adjectives, but we aren’t interested in i-adjectives, we are interested in いー形容詞, and none of these end in い, so it is immediately obvious (and correct) that they can’t be いー形容詞. (That is, it is immediately obvious because we aren’t using romaji.) They are なー形容詞.
And, yes, I know that you don’t see the syllables that way when you use Kanji (漢字), but while an English speaker kind of thinks in letters, a Japanese person kind of thinks in syllables when they hear a word because they have the hiragana in their bones, so to speak. Can a native speaker of an alphabetic language learn to shift his or her thinking that way? I have no idea. But getting away from from romaji as quickly as possible would probably help. How well do you want to learn Japanese? Do you want to just get by and be able to communicate as well in Japanese as you do in your native language? Hey, dream big.
Is my admittedly extremely inexperienced opinion on topics like this any good? I have no idea about that, either, but, as someone still very early in the learning process, I know what at least some of what trips me up, and romaji is one of those things. It might feel helpful in the beginning because it seems to make things easier, but you want to abandon it as soon as possible. And don’t stop with the kana. Move on to the Kanji right away.