We've just had to replace ours but I'm using a Motorola MBP16 and it's got fantastic battery life. But it's bare bones, our daughter is nearly 4 but is on the top floor (3 floor house) and being partially deaf I struggle to hear her if I'm in the garden/ground floor. With a baby I'd look for these features though:
Toddler button, so if the child is mumbling away to themselves it doesn't set off the speaker - allows you to sleep and only be woken when the child starts crying.
Constant mode - allows you to get a constant audio feed from the room, some monitors only activate when the child is loud enough, it's usually best to get there early and settle them before they happens.
LEDs - So you can mute it but still see if your child needs you . I.e. watching a movie, sit the monitor next to the TV and you can still see if they are making a noise. Trust me your instinct will draw your eyes to it.
Night light on/off on the remote , mine only just recently asking for hers to be on, but we turn it off when we know she's asleep, or rather we did on the previous one. The Motorola MBP16 has a night light but for some daft reason you can only turn it off directly on the bloody base unit.
Personally I'd recommend the Tomy TD300 Digital Baby Monitor or any that has all of the features above, I wouldn't skimp on any of them. You'll use this EVERY night so you might as well get one which saves you a bit of sleep. The TD300 is good but be warned you'll need to change the batteries (few quid on ebay) every 12 months.
I wouldn't want a pad for under the baby or a camera personally, unless it's a very ill child there's no need for it.
