I am in particular concerned with legal implications due to strong privacy legislations. However, I was also thinking about performance and false results by users blocking it.
I'm not a lawyer, take my advice as only that. You might want to speak to one about this issue.
Google analytics can slow your page load time, just like every element on your pages does.
As far as false results go, there is not much you can do. The best thing to do is to really analyze the results you get back, over time. However, getting perfect data collection is not the point of tracking software. The software is made to help you make more objective decisions about your website. This is opposed to not having any data at all.
The software is made to help you optimized your content and know what pages work best.
Don't let numbers be the only thing you look at.
Google have always recommended putting their code at the bottom of the page specifically so that downloading the script doesn't hold up the page loading and display. They've also recently introduced asynchronous tracking code which should pretty much eliminate these kind of problems.
I strongly recommend comparing the output of any analytics service to your server side stats. More often than not, you'll find that they don't agree. This is mostly (as you said) due to people blocking JS, or just outright sending GA requests to localhost.
As far as the legal ramifications, I can only offer this advice:
When in doubt, don't.
You really need to discuss this with an attorney, who can:
- Advise against it
Its a unique enough issue to raise here, but you should really seek legal advice from legal professionals :)
You site will be a little slower but it should be relatively unnoticed to most if not all users. Google works very hard to keep their footprint small.
As for legal implications, contacting a lawyer is a good idea. I am not a lawyer so take my advice as is. Most sites use Google Analytics because it is free, unobtrusive, and works well. Also, Google doesn't collect any info you enter into a site just what and where you click.
Another item you may want to consider is whether Google is providing you with enough value to pay for the information you are giving them about your site's data, relevance of specific pages, etc.
They already own a large proportion of this for your landing pages because of their overwhelming search domination, but by installing Analytics you're giving them ALL of it.
Google says that they don't use the information to determine your site's rankings, but they're measurement geeks, and of course they're analyzing the data in aggregate to improve their services.
Like others have answered before, I am not a lawyer.
Regarding performance, Google recently introduced a new async tracking script which makes the performance issues mostly noticeable to the users (obviously it is still using resources, but since the page loads first the user can't tell).
I have tried the new script and it is faster.