The Phenom 2 X4 was never really "the stuff" - it was just a cut-price chip - good performance for the dollar. AMD were happy making small profit margins on their Phenoms to compete with Intel chips at that time. The Phonom 2 X4 never really held a candle to Intel's first generation i7, it was just priced at a lower price point, mind you the Phenom 2 X4 was AMD's top chip, either way, it's plagued by stability issues.
The last time AMD really had dominance was in the days of the AMD Athlon x64 and AMD Athlon x64 X2. The top single core Athlon model (the 4000+) was really dominating Intel's top single core chip at the time, the Pentium 4 (Prescott revision) in everything except video encoding, really. The dual core Athlon was better than the Pentium D in areas relating to power consumption and heat dissipation. Essentially it was this which forced Intel to end its Pentium series and move its focus onto the "Core" line of processors (remember Core 2 Duo) - which was a new architecture based on Intel's Pentium M - which in turn was based on the Pentium 3 microarchitecture, thus Intel returning to computing more per clock cycle rather than increasing the number of clock cycles per second (which generates heat and inefficiency).
Most of you younger kids probably wouldn't know much this far back because you probably would have been too young at the time, but that was AMD's heyday. Before that AMD's Duron processors didn't really match up to the Pentium 3 and after that, AMD never really caught up when Intel took off with the Core line, which evolved from Core 2 Duo to Core 2 Quad and now the Core i3/5/7 lines.