Romaji is Roman letters (English letters) used to spell out Japanese words into English so that people who can't read Japanese can read them. Contrary to popular belief, it is used in Japan for plenty of things. It isn't something you have to learn much to, rather just the sounds of Japanese language (which are supposed to be the first thing you learn when learning a language).
Since romaji is not used to write in Japanese, many people will say that it is not good to use romaji, that it will only ruin your ability to learn Japanese after, that its a handy cap and all other nasty stuff about it.
However, those who can read Japanese don't understand how important romaji is for those who can't read yet.
I am one of those people who used romaji. I started learning the sounds of Japanese then I would write words like "pan" "keeki" "mausu" "konpyuutaa" on paper and stick that all over the items in my house to remember. It is the very first stage of learning Japanese that romaji serves as a good tool to help you.
A lot of people say this slows you down, however I disagree. I think if you were to start off learning hiragana you would be slown down. Hiragana is a pain to read by itself if there is no kanji. Those who think otherwise are people who haven't learned to read kanji yet, and don't understand how important it is.
So if you start off learning hiragana, chances are you don't know a lot of Japanese words (because you just started). It means you really can't tell where a word starts, where a word ends. You can't tell anything, as all you can see is a bunch of characters you just learned smashed together.
You also will be prone to making mistakes in words from the very start. You haven't even got used to making sentences or remembering your first words, you are prone to make mistakes in which letters are in which word. Not to mention when writing all the mistakes you could make.
So for the very start of your learning Japanese it isn't such a bad thing to start learning in romaji as you learn a few vocabulary, maybe a sentence or two before you learn hiragana and katakana and start your journey down the kanji road.
It is also not true that Japanese never used romaji. It is used on signs, food labels and it is mostly used when typing on a keyboard. Most Japanese use romaji input to type in Japanese. Keyboards do have buttons that you can type once and it will give you a hiragana letter, but most Japanese don't use this.
However, there are things you should NOT use romaji for. That is reading full books or wanting to have full on chats in romaji. Romaji is a tool to let you know how to say something out lout. It is not reading practice and shouldn't be used as such. You can already read English letters. If you are at the level where you could read a book translated into romaji, then you are at the level where you should have started hiragana and katakana and kanji.
Romaji does have some problems in it, and that is that sometimes words on romaji websites are not spelled correctly. For example, Tokyo is not Tokyo but toukyou when written in Hiragana. It is important to write the correct amount of letters for each word, or it is spelt wrong and sometimes the meaning can change all together.
Another thing that is completely not true is a lot of romaji books will tell you "when Japanese have a double O sound "oo" the o is always turned into "ou"."
This is 100% false. There is no o's turned into u's, it is simply always that way for that word. Other words may be different, there are plenty of words where there are actually two o's, such as tooi とおい.
The biggest problem with romaji is that you can't tell if words are connected or not. This is especially a problem with words that have a N sound in them.
konya. is this written ko n ya or ko nya?
Not to mention if you stick on only romaji for too long, you will soon learn that you can't continue studying Japanese as most Japanese learning text, is not in romaji. Only the very basics will be in romaji. After a certain level you need to switch. Even further, you will find a point in your studies that you wont even be able to use English websites to learn Japanese. You will have to branch out (like me) on you own and find Japanese websites in Japanese and teach yourself new words.