I've been using 10A432 for the last few days, and I've not noticed any speed improvements. If anything, I find the my MBP to be running a little toasty and the system running a little slower.
This is however, after a clean install. The upgrade from Leopard was terrible, the whole system was beachballin' like crazy. My other MBP with Leopard (identical specs) boots faster than SL and it has more things going on during startup.
I haven't found Finder to be any faster or any rewritten 64-bit app to be any faster. I don't find myself using the new Dock Expose, because I find all all my windows quicker using regular Expose.
Stacks was a joke to begin with, so any subsequent improvement to them is a joke as far as I'm concerned. You still cannot Quick Look files within Stacks or even right click on them. How hard could this have been? But maybe that's a killer feature in 10.7 where Steve Jobs says "And now, we have Quick Look in Stacks with support for contextual menus ... BOOM!" and the fanboys will go crazy and happily drop another $29.
Finder's little slider on the bottom to resize thumbnails and an ability to play videos on the fly is a great feature, but one that is pretty insignificant, and definitely not worth paying for in an upgrade.
Apple has most of its fanboys by their balls, which is why all of us here, and I consider myself somewhat of a fanboy have such massive threads on the new Quicktime icon or the SL packaging. Apple knows it can wet our appetites by the minutest of changes. Which is why Apple can get away by charging $29 for an upgrade that others including myself consider extremely routine (think 10.5.9).
If you look at the incremental upgrades to Leopard, none of them make any real difference to the end user. We're just happily complacent because Apple tells us that the release has bug fixes and is more stable. No new features there.
Snow Leopard could have easily been 10.5.9. The immediate benefits of SL are next to none, which is why it is going to be a hard sell for Apple. The $29 price tag is definitely helping there, but there will be many users who will go home and complain about SL not being different enough on the surface.
The point I'm trying to make is that everything SL brings to the table is extremely superficial. I mean GCD is a killer feature, but it won't be realized until developers start coming out with optimized software which will be at least 6 months away. And if Apple's own apps don't seem any faster in SL, I wonder how much of a noticeable performance gain GCD will bring to the table.
The features in SL are great, but most people will justify their purchase not because of the features, but because $29 is peanuts for most people here. Think about this ...
I'm almost ready to do a clean install of Leopard and go back to my fine-tuned system. And personally, I feel Leopard was definitely not worth the $129 either.