Magento gives you the oppurtunity to log almast everything in the /var/log/system.log file. All datas you usually get by using print_r or var_dump can be logued there.

Mage::log('Message');
Mage::log('Message:' . $variable);
Mage::log($array);
Mage::log($object);
Mage::logException($e);

All these outputs uses print_r() function, so you will easy see eevery array or object content.

If you wish to use your own log file, and don't get messed up with all other logs that system.log can log, you can simply use these syntax

Mage::log('Message :', null, 'mylog.log');

the mylog.log will be created in the var/log/ directory and will be easier to read.