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Very useful for seeing what's going on. The info it shows is great, clear, easy to understand.
You have to do an annoying amount of clicking around to see anything, and can't set defaults, so to see the first page I want every time takes about 8 clicks.
Can't see 2 screens of info side by side. They will let you export data, so you could set up your own stuff. Seems to update twice a day, first about 3 hours before the end of the day with incomplete info and later with a complete days data.
Easy to compare results for 2 time periods. Easy to set up a goal and see which sites referrals are most valuable.
Overall a great value for the price (free).
It's proven to be a valuable, easy-to-use tool -- although I probably wouldn't opt to rely on it and it alone. I continue to use several other Web metrics softwares with features Analytics doesn't have...yet.
I've seen Analytics stats fresher than 1 hr and staler than 4 hrs, but mostly it's right in between.
I haven't perceived any effect one way or the other on site performance.
The only negative thing is that it's been painful to watch over the last month with Google dropping pages, juggling supplementals, fumbling robots.txt, and seemingly self-generating HTTP errors with extraneous snippets of unicode. Google itself hardly shows up in the Analytics search engine comparatives over the last few days.
I don't believe Analytics is implicated in all that -- it's just the messenger.