Five and a half kids here. The only time we've gotten use out of a baby monitor is when we're at my family reunion and there are 40 grandkids running around, ranging in age from 0-18 years old, and the baby's crying would be completely lost in the background noise. Other than that, we've always been able to hear the baby crying--it's remarkable how your ears adjust to pick out that ever-so-faint sound.
If you're going to get one, I'd second the recommendation to get an IP camera of some sort. We have a D-Link 930L (which is a basic, daytime-only version of the one FrugalSpendthrift recommended) at work, and have very few complaints. You'll want a night-capable version because it's darker in the kid's room. And when the kid outgrows it, you can repurpose it for something else.