You shouldnt trust coin mixers, be very careful with them since they can steal your bitcoins and never send them back to you while they are "mixing them"
that's true.. and that's why , when you want to use a mixer service, you should always choose a trusted and reliable service.. be extremely careful when using 
these kind of services..The problem is the type of people who set up mixers. Most I don't think are in it to help people remain unknown. They are in it to make money.
If a government shows up asking for IP logs you know they will be turning it over very very quick hoping to not spend a ton in lawyers.
Also biggest concern is they just stop sending out. Lets say business is slow, they get a big amount to mix. They run away with it and any smaller ones that were sent before people realized it.
Just a lot of possible bad things.
About IP and Goverment. There is 2 sites at BitcoinMixers.com that need no registrations. Using VPN/proxy you get your adress for sending BTC and using TOR send them and got your new bitcoins. Simple, yeah?