I have driven the old A4 (many times) and the new A4 a few times but not the W203.
The new A4 is a nice car.
The only criticism I can level at it, is that it's a bit less special than other Audis. The A8 interior is wonderful, the A6 still quite special feeling, and the A4's, whilst still high quality and in my opinion and many others, the best in its class, feels more 'everyday'.
To drive, it is great. OK ride but no Mercedes, but the trade off is fairly decent (if slightly distant) handling and a good manual change. One of few Audis where it just feels fun accelerating in a straight line. It builds on the strengths (what ones there were) of the old A4 and adds a fair bit more too. Come to the bends and it is willing. Like almost every car today grip is high, but it also turns in well, with little roll and a feeling German engineers actually had some fun when engineering the car, all of which surprised me a little
.
It's also very modern - electric everything, ESP/EBD and all that, indicators that flash 3 times when you just 'nudge' the stalk, but you don't notice any of it after a few miles, just as it should be.
If you're half tempted by diesel, get the 130bhp 1.9 one and prepare to be amazed.
The W203 is supposed to be great too (especially post-facelift).