The first thing to keep in mind as you look at this performance comparison is that the Duron's chief competitor isn't present in a non-overclocked form. Intel's Celeron, currently available at up to 700MHz (66MHz FSB), doesn't even outperform the "slowest" contender here, the Athlon 600. The Celeron 700 scores a 123 under SYSMark 2000, and the 566MHz entry level part comes in at 111.
The entry-level Duron 600 is already as fast as the original K75 based Athlon 600, and the new 750MHz part proves to be around 4% slower than the Thunderbird 700. On a clock for clock basis, the Duron is around 8 - 10% slower than the Thunderbird with its larger L2 cache.
If you're making the comparison to the older K75 based Athlon's with their off-die L2 cache, the Duron is around 2 - 5% slower on a clock for clock basis.
In spite of the fact that the Duron is "only" delivering 90 - 95% of the performance of its older brothers, keep in mind that the Duron is currently priced at around 30% less than an equivalently clocked Thunderbird.
Compared to Intel's Celeron, the Duron offers clearly superior performance, with a competitive price. The only chance the Celeron has of coming out ahead of the Duron is if you overclock the processor using the 100MHz FSB, but if you overclock the Duron the balance shifts yet again.