I purchased a desktop system assembled locally in early 1999 that had an AMD K6-2 running at 300 MHz. The system was used heavily by two high school students, by my wife (a teacher), and by me (an engineer). It was also used to run various Linux distributions (Red Hat, Suse, Caldera) in dual-boot (LILO) configurations with Windows. Today it is running Windows XP Home Edition SP2 that was an upgrade from the original Windows 98 (not SE). It is used almost entirely for word processing with Microsoft Works 4.5 by my wife. It is left on all the time to save startup time and because it has an attached laser printer that can be accessed over our home network from other systems.
Around August 1999 my college-bound daughter purchased a Compaq Presario 1237 laptop (from Radio Shack) that had an AMD K6 266 MHz MMX(tm) enhanced processor. This was more than adequate for her purposes at college. The lower CPU cost offset the higher cost of the far superior (to HPA) active-matrix TFT display and three-spindle (hard drive, CD-ROM drive, and floppy drive) configuration. The system came with Windows 98. It was connected to the college's network using Novell drivers and a 3COM PCMCIA NIC. She still has it and it still works.
In October 2001 I took the plunge and bought myself an HP Pavilion N5425 notebook computer. It has a 900-MHz AMD Athlon. This has been my main computer for the past four years. It came with Windows XP Home Edition and has been upgraded to SP2. I run all sorts of applications on it: Microsoft Office XP (Word, Excel, PowerPoint, Outlook), Visio, Internet Explorer, Firefox, Opera, Borland C++ Builder 5 (C/C++ software development), Scilab, Winamp, iTunes, Musicmatch Jukebox, Windows Media Player 10, RealPlayer, DivX Player, TextPad, Source Edit, Snag-It, TurboTax, Norton Ghost 9.0, Norton Internet Security (AntiVirus, Personal Firewall), Norton AntiSpam, Webroot Spy Sweeper, Java VM, STEEM (Atari ST Emulator), Adobe Acrobat Reader 7.0.5, N4PY control software for my Ten-Tec Argonaut V amateur radio transceiver, DigiPan PSK31 software, and all manner of tools and utilities (some written by me). It all works. The only problems I have had with this HP notebook PC are related to progressive failure of the cable between the display and the rest of the system, which leads me to the next AMD-based computer.
I recently ordered an HP Pavilion zv6000 series notebook with AMD Athlon 64 3200+ (2.0 GHz) CPU. When this arrives, it will become my main computer. My old HP notebook will then go in for surgery (under my soldering iron) to see if I can get the diplay running again. However, even if I can't, I can still use it with an external monitor, as I am doing right now! (Prior to this bump in the road, I was using the internal display as "1" and the external display as "2" in a dual-monitor extended desktop arrangement.)
Based upon my experience (both past and anticipated future), you should be OK with AMD CPUs, at least in terms of system availability. And remember, reliability (MTBF) is only one component of availability, which is what we users really care about. The other major component is repair time (MTTR), which is probably the more significant of the two. Overall, I have had good experiences with HP products over the years. Also, when HP uses AMD processors they know that they are staking THEIR reputation on that decision.