I'm currently writing a 4900 word article about SP1. And I really mean it - deadline is in two days, and here I am reading the forum instead. Talk about procrastination...
Anyway, I've installed it on quite a few machines and did my own benchmarks on both slow and fast machines, and I've read perhaps dozens of other test reports.
Since you're friends and we're all casual here, I won't sugar-coat my opinion.
I'm pretty disappointed with Windows Vista.
I'm slightly less disappointed with with Vista when SP1 is installed.
Assuming it doesn't Fubar your machine (and the reports of it doing so are being very highly publicized way out of proportion to the actual percentage of messed up machines), you can count on noticeable (though not hugely significant) improvements in file copying and moving and network operations, battery life in notebooks, startup and shutdown times, and hibernation (which now causes less weirdness than it used to when the system comes back). Somewhat large strides have been made at improving printer driver compatibility. There's now a solid API for Symantec and McAfee to talk to the Kernel without needing to patch it. There's full support for 802.11 draft N wireless networking.
XP's SP2 was a huge change to its base OS, but Vista's SP1 is more like XP's SP1 or 2000's SP1 and SP2: minor improvements all around but no significantly large new features.
On the OS Satisfaction Scale, calibrated to Windows XP being a 100 (and it is a little strange to say that):
Vista without SP1 is around a 60.
Vista with SP1 is around a 70.
Call it moving from a D grade to a C grade in my book. That's a worthy improvement, though still not a stellar grade.
-Llama
