Newbie questions


I am thinking of installing a burglar alarm system in my home and
have a few questions after I looked at product descriptions of some
available systems online:

(1) Is it really worthwhile to order the monitoring service together
with the security system? I mean, most really depend on phone line to
call up the monitoring company for the alarm. Aren't most burglars
smart enough to cut the phone line before they enter the house? There
are some that provide cell phone dial up, but I wonder if that's really
worth the trouble.

(2) Without the monitoring service, how effective is the alarm siren.
Isn't it relatively easy for the burglar to take the battery out of the
controller and the siren will stop shortly?

Thanks in advance.