I think the s370 Celerons are not that bad for a retro rig. Even though I personally rate the s370 post-PPGA Celerons as a somewhat significantly lesser choice, they can still hold their own. I have (had) 2 retro rigs with s370 Celerons, one 400Mhz (atm non-functional due to it being in pieces after a messy attempt at changing some hardware around) and a Celeron 800.
Even though the similar speed P3's are a lot better, the Celerons have the benefit of having a pretty high CPU clock on a lower FSB. There are P3's available for the 100Mhz FSB, but those are harder to find.
In my opinion, it's the >750Mhz P3's that are worthwhile to get. If you ever need a 750Mhz 100FSB P3, just take one of the common 1Ghz chips and underclock it 😉

I think the post PPGA Celerons are overall about 30% slower compared to their bigger P3 brothers while having about the same power consumption.