I tried it on my older PC. Scores were about in line with what I expected. I can't speak for experience yet, since I haven't tried much, but it was slightly sluggish (not overtly so) just navigating and doing some basic things. To recap, it's a Pentium III 933MHz, 512MB of PC133 SDRAM, a GeForce 4 MX440 64MB AGP8x (AGP4x motherboard though), and a 40GB 7200RPM 2MB cache IDE drive.
The CPU got a 1.4 (would be interested to see if the Williamette Pentium 4s got much better since they weren't really much better, and what did the Tualatin get?).
The RAM got a 1.7 (surprisingly).
The GPU got a 1.9 for Windows.
The GPU got a 1.0 for gaming.
The hard drive got a 5.4, which was quite higher than I expected (expected 3.x to 4.x).
I'm sure if it had a DirectX 9 GPU in it, it'd get the 1.4 as it's base score from the CPU. I'm just wondering why the CPU and the RAM have different scores, since they're both 133MHz, and the RAM is way low at 512MB (in my experience on my main PC, RAM speed and amount are what it measures, and whenever I raised my CPU speed, the RAM score went up with it to match, so I was expecting to see them both ~1.x matched).
The test completed fine, obviously. I didn't run it. I had just installed, and went off to grab some food, and got back and it had done it itself. Maybe I'll try some games on it soon enough.
I'm also a bit surprised the properties menu identifies the CPU by it's string name (Family 6, model 8, stepping 6), rather than just Intel Pentium III or something. I know these are old CPUs, but still.
From what little I've seen, Windows 7 likely won't replace Windows XP on any Pentium IIIs or 512MB PCs (if they're primary PCs), but even for Pentium 4/Athlon XPs, or 1GB machines, I'd say it'd be a good idea. I know it runs well (very, very well, better than XP did on it even) on my brother's PC I gave him, which is a Pentium 4 2.8GHz, 2GB RAM, and GeForce 6800 GS AGP.
I shut off a few services and it's cut down on the memory usage a lot. It's idling at 262 MB used right now.
Besides the obvious search indexing, what did you turn off? I wouldn't mind trying to see what effect this has myself.