They are better than nothing. If you would like to explore using a public counter, you can visit Web-Counter. They are more difficult to implement than public counters. You also must be using a server which allows you to run CGI scripts on the server in most, but not all, cases. They also require that your server automatically generate access log files.
Most servers do.
Not all ISPs will allow you to do this. Some ISPs not only allow you to do this, but provide the software and installation instructions for setting up counters. The counters can be very simple like the public ones, or very sophisticated and advanced in their statistical analysis. They are generally very accurate. If someone can access your page, there access will be recorded and accounted for.