Generally the build is good. Some hard plastic down around the gearbox and glovebox but overall its not bad. No annoying noises or squeaking from the interior. Its solid and no big gap between the materials. Its certainly not much worse than my mothers brand new BMW 1-series. Sure it's not A3 level but you dont pay a fortune for the car so you cant really demand it. For the money ST is great value, saying otherwise is just lack of knowledge.
That was the point I was trying to make to C4L, if you were comparing a BMW to the S1 then outright interior quality and materials used would have more of a bearing.
When you buy a Ford you already know it's not competing on the same level so the expectations aren't the same and it's not like the Fiesta is bad to begin with as you've already highlighted.
Currently there isn't anything to compete with the Fiesta ST in terms of fun factor vs £££, but then again the guy considering an S1 wouldn't have the ST on his radar, he has slightly different wants and demands.