Depends where you live and what sort of house you own really. The majority of house burglaries are by drug addicts and undesirables, who are after quick cash from electrical goods and a getaway in your car. They are not intelligent enough to defeat any standard alarm, and not brave enough to do it whilst you are likely to be home. Obviously thats not true in all cases, and desperation will drive them to burgle a house with the owners at home. In most cases though, an alarm will deter them no end.
If you live in an middle/upper class area, with a home that breathes money though, these are the house of choice for the "career burglar". Any standard alarm will be studied, so they can revise ways to defeat it.
Alot of burglars will do recces prior to the attempt as well. They will knock on doors, and ask for a random person who doesn't live there or similar, just to scope you/your missus out to see if its worth it.
IMO burglaries are pretty uncommon in most areas, but you can only reduce the odds of it happening to you by fitting an alarm. As said above, even a cheap and basic alarm will put off the majority of the undesirables, but if a career criminal wants to gain entry to your home, they will.
