I've got one of these mobo's. For a while, I was wishing I got the P4B533 because of the lack of the PCI/AGP multiplier lock. I'm OC'ing a P4 2.4b to 2.52ghz; if I go any higher the video card chokes on me (Asylum Ti4200 128). But revisiting the specs, there are some pluses to the P4S over the P4B: 3 DDR slots instead of two, 6 PCI slots instead of 3, and IDE UltraDMA 133 support. I guess if you're looking to overclock, spend the extra $6 and get the P4B, but if you're looking for flexibility, get the P4S.
At $49 and $55, the prices are great, but I might sleep better saving money somewhere else instead. Mobo problems are such a pain to troubleshoot...
a2k