The rule of thumb was that Cyrix' processors were fantastic, pound-for-pound, when used in an office environment running Windows and Microsoft Office, but that they were inappropriate for games due to the lack of an FPU and the attendant drop in 3D performance. 'Carmageddon' running on a Pentium 120, for example, was quite playable; whereas the same game running on a '166' Cyrix 6x86 juddered and jerked.
A note about p ratings, as mentioned above by xunker - a Cyrix '166' actually ran at 133mhz, but, when used with anything that didn't require a great deal of floating-point mathematics, was roughly equivalent to a Pentium running at 166mhz.
Nonetheless, from about 1995-1997 Cyrix had a higher profile than AMD, and they kicked off the market for cheap, decent PCs. By 1999, however, competition from Intel's Celeron, and a new range of games-orientated AMD chips - not to mention the lack of development due to National Semiconductor's aforementioned insistence on 'value', and the fact that staff were leaving in droves - led to Cyrix becoming nothing more than a brand name.