My missus has the 1.2 16v Dynamique on an 02 plate. She canes the nuts off it on a regular basis, and I've yet to see it return less than 40mpg. It generally hangs around the 45 mark, and can hit 50 on a good day.
It qualifies as 'nippy' rather than fast. It has a good power to weight ratio that makes it quick off the mark, but while it certainly doesn't struggle at motorway speeds (it'll cruise all day at 80 in 5th) you need to drop down a gear or two to overcome wind resistance if you want good acceleration.
My only real criticism is the standard sunroof: Most of them rattle and creak from new (ask your Renault dealer and you'll be told 'they all do that sir!') More annoyingly, it's a tilt only roof - you can remove it, but it's bulky, cumbersome, and you risk taking chunks out of the paintwork on your roof if you slip.
But it's still probably the nicest small car I've ever driven: Out of the small car's I've driven, probably only the Punto gives it a run for its money. It definitely beats the Corsa, and wipes the floor with the Ka.
Avoid the 1.2 8v at all costs. If the one we hired in Ibiza was anything to go by, this engine wouldn't pull the skin off a rice pudding - if you wanted to do an emergency stop, you just turned on the air conditioning. It left us sceptical about the 1.2 16v, so we took one for a test drive before ordering ours from JamJar. The difference is amazing. If you're not used to a multivalve engine, then you have to get into the habit of taking the needle much further up the rev counter when you want to make the most of the performance, but this soon becomes second nature. Unlike some small cars, you don't really need to thrash it like that all the time - just when you want to get the most from it.
Diesel will probably make more sense if you're doing above average mileage or buying second-hand. Otherwise, your fuel savings may never offset the original purchase premium that they charge for the
burners. List prices for a 3 door Dynamique without A/C (from TuskerDirect's website, because Renault don't currently list the 100bhp model on their own site!):
1.2 16v = 9407
1.5 dCi 65 = 10127
1.5 dCi 80 = 10627
1.5 dCi 100 = 11327
Based on the manufacturer's figures, the 1.5 dCi 80 uses about 70% of the fuel of the 1.2 16v. To save the £1200 purchase premium (assuming both petrol and diesel are £1 per litre) you'd have to buy 4000 litres of petrol for the 1.2 16v vs 2800 litres of diesel for the dCi. That's around 600 gallons of diesel at approximately 70mpg, or 42,000 miles to break even.