I used WassUp stats for a good year and a bit. I wanted to try out another statistic plugin; after doing some research, I tried out SlimStat.
I must say, when comparing, SlimStat goes much more in depth with your traffic. When visitors come to your site, the plugins seem pretty fair, but when it comes to managing what they've done on your website, SlimStat is very impressive! (It shows what link they clicked on your website to go to your next post etc.) -- Not just show "they went to that post", but shows what link they clicked to get there!
Also, the features within SlimStat are exceptional. Tons of different alternatives to track your vistors to help you bring better content to them by seeing what they've searched.
Another thing to mention, is the GUI of SlimStat is great. At first I was a little confused (but this is for every plugin to get used to it), but when I started seeing my traffic being tracked, everything because very easy to understand!
Great job SlimStat, I wish I changed to your plugin earlier! ;)