My DS is coming up to 2 years old and we have found the monitor indispensible! We still use it in his room every night and during his afternoon nap. His room is at the front of the house, our bedroom is at the back - during the night we'd never hear him if he cried, had a stuffy nose, or whatever. Same when we're downstairs when he goes to bed at about 7:30pm, we usually watch a bit of TV later in the evening - would definitely not hear him cry without the monitor. We have a Tomy Digital (sound only).
Have now bought another monitor for baby 2 so that we can continue to use one in DS' room! Bought a Tomy video this time because it was cheaper in an end-of-line sale than the audio-only ones! Have tried it out in DS' room and will miss being able to see him when we go back to his original sound monitor!!
When i'm home on my own, I have to put DS in his cot with toys when i have a shower (no way I could leave him wandering the house!), so i take the monitor in the bathroom then. We use it so often we couldn't be without it. It also has a temperature display which I don't use religiously but it gives an idea of whether the room is too hot or too cold.
Over Christmas, DS was unwell with a cold. Before any other symptoms (ie. we didn't know he had a cold/chesty thing), he was really short of breath at about 1:00am and he got quite distressed. Without the monitor on in our room, we'd never have known.
It's everyone's individual decision, and does depend on the layout of your house and whether you'd hear LO without the monitor, but we certainly wouldn't and for me it's been essential. Having said that, I don't think you need top of the range. I'd go for mid-range (£50 ish).