we have one that can run on batteries as well which I find useful as you can cart the parent unit around if you want to go outside etc. it also shows the temperature it also has a nightlight which goes on for a couple of seconds so you can see the temp.
those are the bits I use
it has other functions, like playing tunes, that I dont use
We have the BT digital plus one which was coming top of all the reviews when we bought it and is still going strong 2 years later.
We wouldn't be able to hear DS from elsewhere in the house without it. We also needed it quite quickly because although he was in with us for a while, he went up to bed earlier than us so although he was in our room, we were not in bed at the same time as him, we sneaked in later. He also went into his own room relatively soon.
Another vote for the BT digital Plus. Whilst we didn't use our at night when LO was in with us, I did use it if we were sitting in the garden as we couldn't hear him that far away and also on holiday sitting out in the evening. LCN
Unless you're going to have your LO in an upstairs room for their naps from day 1, wait until you need one.
We went for a really basic one because we only have a small house but I like to hear the first snuffles of waking rather than the full-on crying as I find it easier to resettle DS that way. The only thing I'd do differently if I was buying another one is get a digital one, ours gets interference from our mobile phones, wireless internet, and flatscreen TV and can be quite noisy.
If you do buy one before the baby then plug it in and try it out - we bought a highly recommended one but our walls must be wrong and the signal was lousy etc - so we had to return and try two other ones before we got one that worked. Also we only used it for the first year after that you can def hear the little ones.