HI i think it's pretty accurate, have one website that has 200 visitors and statbrain tells med that i have 188. Better than most off the tools out there.
Yes, statbrain is generally more accurate than most as it is able to aggregate details from multiple sources to get the number.
It's true that statbrain isn't realtime. I don't think it can update very often because it relies on various other sources and cannot overload their systems with all the requests.
Statbrain used to be very fast and accurate. Right now the website is almost unreachable and very slow. Try CalcuStat.com. it measures the number of visitors, the number of pageviews, website value and even show similar sites.