Wow, I just loaded this onto my SD870 IS and it's great! Based on my early impressions i would highly recommend this to anyone with a Canon p&s. It took me maybe 10 minutes to get everything going by following the CHDK faq.
My favorite features so far:
-adds a bracketing function that the camera previously didn't have
-greater range of manual shutter times to select from
-ability to shoot in RAW
-better battery meter
-live histogram and DOF information
-"Golden Rule" overlay grid to aid with composition
I'm going on a ski trip this weekend, I can't wait to put some of these new features to use. It's like getting a brand new P&S!