OK. So let me first stipulate that I'm a very experienced WP user and admin. Most would consider me "an expert," though I know there is much I don't know, and I'm no programmer. This is an unsolicited response to the grotesquely unfair, inappropriate and inaccurate slams against a truly excellent piece of software.
That being said...
Of all the free stat plugins I've tried (and I think I've tried them all), SlimStat is the best. Period. Bar none.
If there is a better out there, I want to see it.
IMHO, SlimStat is better than most of the paid stat plugins/services, and I've tried a ton of those. Most of 'em are grotesquely overpriced, particularly when you see what SlimStat delivers for nothing. Nada. No cost.
I use SlimStat on over a dozen sites now. My team and I love it. Not only does it work, it does it's job without conflicting with other plugins! That may seem like "so what - it's supposed to do that" kinda statement, until you realize how flaky plugins can be - and how often even expensive paid plugins do not "work and play well with others." I deal with plugin problems regularly. But I can say categorically that SlimStat has not been one of the problem children. Not even once!
IMHO, WordPress ought to scrap the stat app in Jetpack (which I think is really weak) and recommend SlimStats instead!
It's true that one man's great stats app may be another's POC. Each app calculates "views" and "visits" and whatnot by different criteria (as does Google Webmaster Tools and Analytics and all the rest), and the way a particular app does it may not be to a every person's liking. I mean, I've spent days trying to reconcile what Automattic's own app (WP Stats, in Jetpack), Google and my server - all at wild variance to each other, because of differing methods of quantifying the same data - were telling me. Can you relate?
But in terms of instant quality data, usable metrics, keyword and search info, and everything else you need, I know of no other free stats plugin that even comes close!
SlimStat does it all, and does it very well. Again, there are a few very nice paid plugins/services that do the same job - but you'll pay out the nose for them. This very capable plugin also uses far less clock cycles than I would expect. I've got a few plugins here and there - that I need for specific purposes - that don't do half as much as SlimStat and use two, three, even five times as much processor power. Gah!
As far as I'm concerned - to the moment - there is nothing else out there I'd rather use than SlimStat. I don't know how the developer continues to provide such a frankly amazing plugin for no cost (I really don't), but I commend him/her/them. It's a truly excellent piece of problem-free software.
SlimStat flat out rocks.
My two cents.