I love St Pancras, but it does have some quite major flaws.
I have travelled on Eurostar twice in the last two months and the Eurostar passenger experience flows very well (ironic that I am posting this this evening as 373's are dropping like flies!).
Regarding EMT and SET being on the fringes, I think it depends on where you approach the station from. I normally approach from Kings Cross and so enter by Starbucks and for me this is the "main entrance" and so I regularly use shops in The Circle, and find it easy to access EMT, SET and FCC, with EMT and FCC the operators I use most often at StP.
The new Northern ticket hall will position more customers into that Starbucks entrance.
I think the positioning of Starbucks is appalling, that Starbucks, or St Pancras has not made some effort to protect customers from the elementss, even during the summer thats a cold place to sit and wait for a train.
The customer information systems are wanting and appear to have little co-ordination between the four operators, it would be intresting to see how quickly SET passengers facing disruption would be directed to FCC or vice versa.
The management at the station is poor, and again uncordinated, with Network Rail failing to take the lead that they take at other major stations, leading to a disjointed poor culture where staff are grouped into their operating groups and don't talk to each other.