Well as an FRS owner for the last 3 years and 25000 miles and having a ST on order I had a 25 mile test drive in the ST on Thursday, here are my thoughts.

Took the FRS up and had a decent run in the Electric Orange demo car, I did a direct comparison as I covered the test route immediately afterwards in the FRS before heading for home.
Covered A roads, B roads and round town.
First impressions, well it's better behaved around town than the RS.
Seats are very comfy and supportive.
Dash is clear and well laid out.
Clutch and steering are a bit light, didn't try and change the steering setting.
So first GLF sign in sight, 3rd gear and........what a noise, the engine is silky smooth but when revved the warble is addictive, not overly loud but just enough to put a smile on your face.
Do not expect FRS loud, it's not like that.
So up through the gears, the box is good but just lacks the feel and precision of the FRS box, might just be infamiliarity but I just found it a little bit too vague for my liking.
Overtook a few cars to test the in gear stuff and it is very good, doesn't feel as brutally fast as the FRS it's more of a progressive acceleration.
Trying to think of a way to compare the acceleration response in both of them, the only way I can think is this...
In the ST you put the foot down and it's gggrrrooowwwlll in the RS it's ROAR !
Into 6th at 70 and the car is utterly refined at cruising speeds, very comfortable and composed on the road.
You could cruise a long way in comfort.
Turned off onto a familiar B class road, this is where the compromises started to show.
Just felt a bit vague, still loads of acceleration available for the punch out of the corners and the brakes are very good, stopping the car very well.
It's just the bit inbetween, the weight of the 5 cylinder engine and steering combine to make the turn in just not as sharp as the RS.
While the steering does tell you what is going on very well, it's just not as good as the RS.
Best description would be if you ran over a 10 pence piece in the RS you could say it if was heads or tails, in the ST you'd say I think it was a coin.
They behave very differently in the corners, where the ST is a bit softer with a heavy nose, a very slight delay between steering/accelerating with a then slow lazy feeling push of acceleration, the FRS comes alive, responding instantly to any input and feeling thugishly quick through a set of tight S bends.
It was about 3 miles down the B road that I was aware that I was gripping the wheel just a bit tighter than I ever have with the RS.
I'm not saying it's bad, just that it doesn't have that utterly solid feel with tons of feedback that you get from the RS.
Which considering that overall the ST feels more solid and well built than the RS does feel a bit weird.
Back onto an A road and again the ST shone, a quick detour onto another more familiar B road just confirmed that the ST wasn't totally comfortable being thrown through tight twisty stuff.
A final A road cruise and it was time to hand it back.

So in summary it is an excellent, very refined, silky smooth car with a horny soundtrack, but FRS replacement, no.
After driving it down the B roads I just carried on, in the RS I would have turned around and driven it again.
The billion dollar question, would you trade in the RS for one?
If it was the only car I had and I needed a more practical car then yes, but I don't so no, if I had personally traded my FRS in for the ST I would have regretted it.
Some other observations, the handbrake position could have been a bit better, all the controls are well laid out and have a good feel to them.
The steering wheel is nice and tactile, with a nice soft leather feel.
Bit annoying that you have to lift the centre arm rest to put the seatbelt on as the buckle is buried underneath it, might just be me though with the seat well back, maybe with practise/familiarity it won't be an issue.
The build quality is well ahead of the original Focus.
Again might just be me but when I got out I found it a stretch to clear the side skirts, didn't notice it the first time I got out of one, but worth noting all the same.

Can't wait for mine to be delivered now.
The car is going to be an absolute belter and anyone who buys one is not going to be dissapointed (unless they've expected a direct replacement for an FRS) overall as an all rounder not a specialist RS this is the best car Ford have ever built.