Coming into this thread late, but thought I put our story in too. I don't know if ABA is the answer or not, but this is what ABA has done for us.
DS was diagnosed with classic autism (childhood autism) at 2.5, completely unexpected probably because I had no idea what autism was and DH although knew autism existed, but actually did not know what it meant. Our HV got the ball rolling for us as DS was very difficult and she was coming to visit DD for her usual check ups after birth. I refused to accept the diagnosis and said to the paediatrician they got it wrong. It took about 5 months for us to get a diagnosis. At the time, DS had about 20/30 words the most, but could count and knew all his colours. He did not eat much (only had juice) and his weight was at the bottom 10% for his age, but the GP refused to give any supplement because he was quite tall. He behaviour was very challenging, he did line things up a lot but not obsessively, he was up every night, although we had DD we were trying to attend his every needs, so DD never had me, DH was the mummy to her. Despite getting an early diagnosis we got no help to improve DS's speech, the course we were sent on were below DS's level, so after two of these courses which involved us taking time off work, we got two reports to say that DS was not suitable for those course, he needed to go to higher level courses. We as a parent did nothing to help him either as I did not believe that he had autism and DH did not know what to do (I did not know about mumsnet then). By this time DS was 3.5 and he was refusing to play with other children, not eating, not sleeping, did not acknowledge his sister, so clingy with me that I even went to the loo with him day and night as even night time he woke up if I moved from his sight (he was sleeping on my bed while DD and DH slept on there own).
We moved out of the UK when DS was 3.5 and on the same month we arrived here, I saw an advertisement of somebody who was offering ABA. Being in denial, I had no idea what ABA was ( I did no research into autism I am ashamed to admit now), but by this time I was recognising that DS has serious speech delay as he stayed at the same level of when he was 2.5. So we started him on ABA, 10 hours a week as that is what we could afford at the time. After the initial 6 months, he started eating the same food as us. Although his communication level stayed the same, his behaviour improved dramatically. His separation anxiety was nearly non-existant, he started picking up more words everyday and within 8 months of starting ABA, he was asking us for things in sentences with a please at the end.
DS is now 5.9. He behaviour is much better than our NT daughter, he can communicate very well, although still does not have a conversation. We increased his ABA hours to 30 hours a week because we saw the huge improvement in his social behaviour. He is friendly with an adult (although does not understand the social boundaries), he is comfortable with other kids still playing alongside as he seems to have problem continuing a conversation with them. He can have a one sided conversation, ie, I can ask him questions and he will answer, but he does not participate by asking anything. He still does not understand why questions, but we belive that if we can continue with the ABA, he will get there soon.
DS never had any sensory issues, so taking him out to noisy places was never a problem, but our problem was to make him behave, he would do a runner thinking it's funny, he did not understand the danger. But with ABA, we have been able to get rid of a lot of non-desired behaviour. Only thing I have to say is that in the last two years his speech has not improved as much as we were expecting or hoping for, but his receptive understanding is very good now. He is also learning to read and write quite fast now that we are focusing on education side (we did not try this before because we wanted to deal with challenging behaviour first). It also helps that DS probably does not have any difficulties as he can learn things pretty fast. He still seems to need that one to one to learn, but he is getting so much better at learning in a noisy place or with disruption.
In my minds I have no doubt that DS is where he is now because of ABA and one day he will be able to live a normal life in the NT world because ABA is teaching him the skills. He is still language delayed and still has a long way to go but he will get there and when we go back to the UK if we have to remortgage our home to continue with ABA we will do so without any hesitation.
Sorry for such a long post.