The Celeron 1.2 beats ALL the Duron and original Athlon processors simply because it supports SSE, period. Even the Athlon XP 1500+ has poor SIMD performance compared to the Celeron Tualatin.
1) Athlon (T-bird) compensated for lack of SSE with a <i>very</i> powerful x87 FPU--strong enough to tear apart a P3 non-Celeron any day. It's a fine way to compensate, especially seeing as SSE's ultimate purpose is to speed up certain floating-point ops.
2) AthlonXP has a very good SSE implementation. Ace's verified that by benchmarking with an app called KribiBench--an app so SSE-optimized, it absolutely <i>would not run</i> without some form of SSE features in the processor. The Palomino just about kept up with a P4 Xeon, even with a 500MHz speed deficit.
Your point about heat, OTOH, was certainly valid back then. The Pentium3 was a much less power-hungry chip than the T-bird or the AthlonXP. Unfortunately, the Pentium4 is now a power hog (more so than any Athlon), and Banias (a revamped Pentium3 for low-power settings) has about the same heat/power characteristics as the low-voltage AthlonXP.
Right now, I'd say the main disadvantage with going for AMD is that they're not really <i>quite</i> at the top anymore, despite the PR ratings. The PR ratings were once valid figures (even very conservative), but lately they're slipping into the "fudged" region.
For low-end or midrange systems, though, you just can't beat the performance/value of an AthlonXP+nForce2.
Compatibility problems for the Athlon may have existed at one time, but they're pretty much nonexistent now. We largely have the nForce2 chipset to thank for that.
Thermal protection problems are a bit more significant but not major. It's reached the point that decent thermal protection is a basic staple of any new AthlonXP motherboard.
<i>I can love my fellow man...but I'm damned if I'll love yours.</i>