I upgraded my Macbook Pro (mid 2009) to 10.8 about a week ago. Just a few observations:
When waking from ‘sleep'(when the screen is shut) the wifi takes much longer to connect than in previous operating systems. I started to time it, over 20 seconds from when its opend to when I’m back online. I’m not far from my access point or anything, it does it when its 5 feet away from my Apple Time Capsule. I never timed it in previous operating systems because it was never an issue, but it’s definitely slower.
Also I’ve noticed the battery life seems to be shorter. I haven’t done any tests with that, but it certainly feels like I’m plugging in more frequently.
Guess I’ll wait for 10.8.1.