After much number-crunching and comparison - in particular with the VW Golf bluemotion, I rcently bought a new KIA Rio '1+Air' 1.1 CRDI.
Following over 4,000 miles of driving, I can conclude:
Basically, if you keep the top speed to 60-70, rather than 70, and just use the highest gear possible (keeping the revs below 2,000), you can really push up the MPG. Idle Stop & Go is really good for saving fuel. It works by turning off the engine when you put it in neutral, and then take your foot fully off the clutch. It immediately starts the engine when you halfway depress the clutch again. The engine startes so quickly, that it's running before the clutch is fully depressed, and always before you're even moved the gearstick - it's pretty awesome. If you consider that the engine idles at 1,000 revs/minute, waiting for the normal 60 seconds is the same as driving around an extra half mile, so on a 5 mile (10 minute-ish) drive, that's a 10% saving.
ISG does have it's qwerks though - if the battery isn't charged enough it doesn't engage (a little light shows it's disabled), and will actually restart the engine in ISG mode if the battery charge drops to low (e.g. the air con or lights drain the battery) - as you're in neutral, this isn't ever a problem. It also (cleverly) restarts if you start to roll the car, presumebly so you've full power for the power steering etc.
All in all, I've found it a really good car, it handles well, is 'quick' (though not blindingly so), looks good, and is what I'd have hoped for in a car. I got 5 years of servicing (for £500), and combined with the 7 year / 100,000 mile warranty, ZERO rated car tax, and ZERO congestion charge band, I pretty much only have to fill it up once every 500+ miles (tank is 40 litres) and forget about it.
The only minor point is that they dealer only pumped the tyres up to 32 PSI, which is correct for the non-ECO tyres, but the ECO tyres are meant to be 38 PSI - when I did this my MPG jumped up by 5 MPG, so this really makes a difference.
The only thing I would change would be the fact I got steel wheels, because the '1' model has these, and it also has the best combined MPG. If I knew the MPG impact (positive or negative) of having their alloys, I would have almost certainly got their georgous sporty ones !
If anyone from KIA is reading this, I'd gladly trial alloys and publish the results here !