PROS
- sometimes board level optimizations that make them amazing, my old IBM PC 330 100DX4 486 box was like that, it could even boot off ANY drive in the system like it had SATA, and it performed more like a Pentium 100 on the stock Intel DX4-100 with a lot of RAM.
- Proper thermal subsystem for the hardware, everything is exactly in the best place it can be for thermal I/O - this becomes a big deal on the Pentium III and higher systems.
- If you have the matching hardware things feel just a little more "offical" - nothing felt more official than that PC-330 with a G40 monitor with digital controls, IBM Win95/98 keyboard with palm wrest, and a geunuine IBM PS/2 scroll mouse.
- Sometimes the company STILL has all the support documents and files for it. Compaq, Dell, some HP, and some IBM Systems are extremely well supported and documented online still, compared to perusing TH-99 for jumper settings, maxiumum RAM/L1/L2/CPU Type and so on.
CONS
- Underpowered and proprietary power supplies. Usually have about 65-200 Watts at most, verses as much as 350 watts for 8088-80486 era systems, and above 400 watts for anything after if you run a lot of crap like I do. Certain brands like Dell, HP, and Compaq had proprietary PSU that would be difficult (Compaq Deskpro 8086/286/386) to impossible (IBM PS/2 series) to find replacements for which means you'd need to delve into the world of electronics and power supply repair to actually continue to use the computer if it burned out (assuming the problem does not involve rewinding a transformer).
- Tiny cases, some are so small they have bad cooling, others are hard to find space for more than one hard disk and one floppy drive in, some require extra mounting hardware that may not be in the case because the option for it was not installed when it was originally built.
- Proprietary Cases - Can't replace or upgrade the motherboard, can't fit another type of PSU in it (they often go hand in hand), sometimes devices like the IBM PS/2 systems are totally special to that genre of IBM Compatible.
- Limited upgrades, typically they picked 3-4 compatible CPU at most, and a handful of memory configurations with a lower than usual ceiling (ie 32MB instead of 64MB on a 486).