I absolutely love mine, we use the angel care one and it really does work. There is no smell from it and its really easy to empty and wipes clean too. I have mine in my livingroom but we are considering getting another one for upstairs too.
I dont think the cartridges are that expensive its £15 for 3 of them and each one lasts about a month for us so £5 a month really isnt that bad.
The only issue I have with the bin is when LO has made a very messy nappy you can end up getting some on your hand when pushing it down into the bin but we got around this issue by putting a sandwich bag over our hand.
I think its unlikely that a toddler would be able to get into the bin or touch/get the nappies but I dont have a toddler yet and I know my friends toddler can get into everything so I would recommend keeping out of reach or in a cupboard that is toddler proof.