I use it on a decidedly low traffic server - around 25k uniques per day in total, around 200k pages views a day. No issues whatsoever here.
Kind of a hassle in some areas. If you decide you want to change the config, even in a minor way, you have to change the configs for every site. Including adding new search engine matching patterns, eg, adding all the ips that = google hits. That has to be done on a site-by-site basis.
With that being said, it has decent reports. Some things take getting used to, like most reports show "accesses" and not uniques. It's also very cheap, compared to major players like deepmetrix or webttrends.
edit: pasted the wrong number under page views per day
