I had mine yesterday and I have to say, I was not excited for it. I wasn't nervous like I was for my 12 week because I could feel baby moving and so on, so I had a feeling that at least baby was still alive in me (which was my big concern with the 12 week one).
On the other hand, my OH was really nervous and did not enjoy it at all, whereas, he had enjoyed the 12 week one and was not really anxious about it.
I think it is also a bit boring because it's all measurements and checking that all the organs and bones/limbs etc are in place and so on, so I found it less interactive somehow. The sonographer did tell me what she was doing so I could understand what I was seeing and it was lovely to see baby again of course.
I did want to find out the gender and chose to do that. It was quite nice because as it as at the end it was a good note to leave on.
I personally don't know anyone who has had issues show up at their 20 week scan. Regardless, things that can show up at problems with the major organs like the heart, the bladder, stomach and so on as well as the limbs and spine. The growth is measured again and charted. If something shows up as potentially abnormal, you are informed by the sonographer. As far as I understand, you would be referred to a consultant at that point and they take it from there.