i'll try to help a little. first of all it needs to be installed on the server level. so you have to have a dedicated box.
i've installed and used it at my previous web development company and I liked it. it has lot's of charts and graphs and gives statistics on about everything you would want to know (again, this depends on which product you are getting, i think webtrends offers like 2 or 3 different lines depending on size of business. We used the corporte edition) honestly, though, i wouldn't buy it myself. just a little too pricy, unless i had an ecomerce site like amazon (well, maybe not that big but you get the idea)
as the previous person said, there are a lot of free solutions out there, plus if you are not that big or are just starting out, i have found that even webalizer that's in Cpanel works just fine.
they do let you download a trial. so try it out. takes like half a day of less to set up and get running. very easy to do.
hope that helps.
oh yeah, i think it's a windows only thing...unless they've developed or ported a unix version