It's an absolutely good series to watch from the first season itself, with every episode it gives you a lot of thrill. All the actors were outstanding especially the role of the Professor and Berlin.
The only thing which was not needed was the storyline after S2 which I found unnecessary.
Although, S3 and S4 were good but the basic plot behind the show is Heist and they totally forgot about it after S2. If the makers had finished the show within 2 seasons then it would've been considered as one of the best series of all time as the show was purely based on the Heist of Royal Mint of Spain in S1 and S2.
Now , there are rumours or some news regarding the renewal of S5 and S6 of the show by Netflix so let's c how it goes.