AMD is the choice for most budget gamers and overclockers for their Duron line.  This is due to the fact that it is by far the cheapest, most overclockable proc (processor) available in the market today.  For those in that category with some extra money, Athlon is the definite choice.

The features of both Athlon and Duron Procs are about the same, the only difference being the size of the Level 2 cache and some additional instructions for Athlon's 3DNow! feature.

Building a system using AMD procs is less painful than most people think.  People used to think that AMD procs have a lot of compatibility probs, less support from peripheral manufacturers, blah, blah, blah.  But all that's changed with the introduction of the VIA KT133 chipset by AMD's long time supporter, VIA.  The KT133 has support for AGP 4X (Absent in the previous incarnation, the KM133), better power management, multiple USB ports, & so on, & so on.

Cheap Proc means that U can use that extra cash to get some better components for your PC.  A better graphics card or more memory for instance.

KT133 mobos has no built - in graphics, only built - in sound, which can be disabled easily in the BIOS.  SIS & ALI are coming out with a chipset with built - in graphics, more on that later.

Components for AMD systems are the normal ones U'd use to build PCs, no weird or hard to find ones, or expensive ones, such as the RAMBUS Dram.  AMD's using their own DDR (Double Data Rate) memory.

Compatibility probs.  This is nothing strange to AMD systems.  Sometimes the RAM won't work, or the TV card U bought won't have any sounds.  Even the power supply sometimes refuse to power the system up.  Things like that.

Cheapo procs are often viewed a low quality alternatives.  So some people might shy away from choosing low quality, cheap procs.

Reputation.  Intel has a large following in the computing world.  Some people bought Intel systems just because it's a well-known brand.

Support from systems manufacturers, OEM manufacturers, computer vendors, peripheral manufacturers, software developers.  Intel has more of that than AMD.  So people are worried that the peripheral they bought won't work on AMD systems.