Here in bitcointalk a lot of members attain the Legendary status.
Yes there a lot of them who is a great help to a certain site.
But should we trust them in every thing they say?
Look at their trust first

...and also to his "activity" here in the forum ( the last post, the topics , etc... ).
No, activity is a very poor indicator of a person's ability to be trusted. The activity is only an indication of how much time they have spent here. If a person who was going to scam has spent a long time here then he will have had a lot of time to study the market and what people will and will not believe and what they will and will not invest in.
If you are referring to quality of posts then it will only be an indicator of intelligence, and there are plenty of smart people who are scammers