Let people have their opinions, ¡t is not like he/they are saying something outrageous. We are signing a central midfielder who wouldn't get in to the absolute top clubs and never got a cap for Spain. Madrid and Spain preferred Isco and Modric is better than him, at Bayern he would be their 5h best option as an offensive central midfielder after Thiago/Martinez/Schwein/Kroos, he wasn't wanted and wouldn't get in to Barcelona and so forth.
It is fair to say this signing is a great signing for a club who are struggling to keep up as a top club in England and wants to get closer to the top 3. This signing achieves this, but the likes of Bayern/RM/Barcelona are not going to be very bothered by us making this signing, Herrera's quality isn't at the very top level.
Now I personally believe Herrera has stagnated a bit since being moved to play as an AM and if we either play Herrera and another CM who alternate going forward and backward, or let Herrera take the defensive box to box role then that is a stroke of genius. If Herrera masters the defensive box-to-box role I have no doubts that he is the best DM in the world considering the attributes he has.
He has the tackling and positional sense to do it and his physicality will allow him to get up and down the pitch well, then of course his offensive abilities are unbelievable. As a pure CM, who is covered by a defensively minded midfielder I don't rate Herrera at the level of Modric/Kroos/Thiago/Isco/Wilshere/Martinez/Schweinsteiger and so on but he is still a great step up for us.