The M3 will run circles around an S5, but if you're not interested in that level of performance, the S5 will give you style and comfort in abundance.
In some ways though, I still feel the S5 is an odd bird. It's not any more comfortable or better looking than an A5, but it's performance is not remotely impressive for it's price. I suppose the market is for middle-aged+ males who want the style, comfort and convenience (quattro) with some sense of performance - as opposed to many of the people on this board who would start with performance and then want style and comfort.
135i, SGM, Coral, Sport Package, Auto, Premium Hifi, USB/ipod, Apex EC-7s, PPK Stage II