That's great to hear! I was a little skeptical about El Capitan at first, but from what I have heard from others, it is (for the most part) working just fine and running well as well. I never touched past Mountain Lion, especially on my HDD... Mavericks was slow, and I expected Yosemite would be as well.
I have a 1,1 as well (3Ghz model), and I am also surprised at how nicely it runs. This machine feels a peppy as a brand-new budget computer w/ an i3 or something. However, what it has hidden behind its responsiveness is muscle and raw power. Sure, it may not be as responsive as a nicer MacBook Air (although graphics-wise, it's better than that integrated crud), but it delivers a bigger punch. Even if someone compared a Xeon HP workstation from 2006, it wouldn't be as great as this thing, it would only match it in sheer specifications... and we know that specs (especially just processor speed) aren't everything! (But, it seems like some Windows fanboys don't know that) So, I guess, once I get an SSD in my system (and once El Capitan is released--hopefully working as well [or better] than this past beta), I'll upgrade this thing to El Capitan.
By the way, I am interested in Windows 10 on a separate partition. Are you running the 32-bit or 64-bit version? I used to have Windows 7 on this Mac Pro, and I recall it being 64-bit, but when I recently wanted to put Windows back on it, the Bootcamp files for 64-bit Windows weren't compatible with my machine--no surprise. I put 32-bit Windows 7 instead, and all the drivers are applied correctly; however, it does have some lag spikes, which is why I am interested in a lighter, nicer, Windows 10. Were the Bootcamp drivers for the 1,1 able to work with Windows 10 just fine, or were there any hoops through which you had to jump?
Thanks for sharing your valuable feedback on your machine and El Capitan,
-MDD