I've had some experience with the S60, it's quite a refined car, with a good deal of kick. If there's a mild criticism, it's that the back isn't as big as the S70, otherwise it's a quantum leap overall.
What are your priorities in a car? If you want space, go for the 9-5, and forget about the A4. If you want pace, the Saab is probably best, but the margin between the three is so small, it's relatively insignificant. Overall...
I'd say go with the S60, I've had runs in the 2.4 and 2.4T. The 2.4T is probably the best pick of the range, the T5 is fast, but the 2.4T is better value, and probably a more useable engine. The 5 cylinder engine has a great growl, the car is very solid, has good face level air vents in the back, is very refined on the freeway, has a deep boot, and some of the world's best front seats (Volvo and Saab probably make the best front seats in the world), to name a few good points. Basically with a Volvo you get a very liveable car, a car that is very easy to live with day to day - you get to notice all the little tools and features that make life easier over time.
Really, you have to drive the cars yourself. I think if you look at some of the responses here, they don't speak totally of rationality - If you say get this car, tell us why? I'll declare my bias, I'm a happy Volvo driver