I used to hate 'corrections' from people, often that were not listening to what the sensei was saying. However, having quite a new club, I've found it invaluable for my longer term students to offer advice to the others. I think the level of advice should be fluid. However in order to advise someone you must be:
1. confident this is what the sensei is trying to transmit
2. confident that the person being advised is not understanding what to do (and it isn't just a stylistic interpretation or limitation of their body)
3. not overwhelm the person you are advising with information (sometimes advice exceeds practise - which negates any use of the advice).
This could be summarised by only advising when you are sure you can help them, and not to show off how good you are.
Incidently after or during training with a partner I usually prompt them to tell me how it felt - without feedback, how can we know how well it is working?