Contrary view time - it's my lunchtime, so I can afford to be contrary - there are two separate expressions here:

1. Someone is on form.
This usage is quite normal in Britspeak parts of the world.

2. Someone is in great form.
This usage means something completely different from 1. but is semantically correct within its own context.

3. You wouldn't, however, claim that someone is on great form, just on form.



The idiot also known as Capfka ...