The Thunderbird was nice, but it was more of a price/performance winner than overall performance. A 1GHz Thunderbird ran stable at 1.3GHz and was similar performance to a 2GHz Pentium 4 at a fraction of the cost (particularly as the P4 required RAMBUS DRAM, so you could stick twice as much DDR in Athlon for the same money). It wasn't until the Opteron that AMD really started winning on performance. The integrated DRAM controller was a big win and being first to 64 bits (which, on x86, means more GPRs, sane floating point ISA, and PC-relative addressing) gave them a huge advantage. Unfortunately, they haven't really been competitive since the Core 2, except in market segments where Intel intentionally cripples their offerings (e.g. no more than 2 SATA ports on the Atom Mini-ITX boards to avoid competition with the i3 boards, making AMD the only viable option).