For those not familiar the baby mee for bakfiets is a metal piece you install in box bikes for carseats to "click" into. I use the expression click because that is what car seat bases do in US cars. But in all honestly there is no clicking.
Prior to having the baby mee we would use tie down straps to tie the carseat in the box, underneath the carseat we used a blanket to muffle any vibrations. This was an effective and cheap method. we felt like the seat was secure but the luxury of easily pulling the carseat out or putting it in with a sleeping baby was lost. Also we would need to remove the carseat every day because it wouldn't deal well with the outdoor elements overnight.
The baby mee cost us $105 (US). The set comes with "instructions" for installation printed on a sticker located on the side and also includes all the hardware needed. For wood bakfiets you drill the base plate into place. In the Urban Arrow's metal perforated base you just choose the holes to bolt it through. The base plate can be locked to the removable metal frame that holds the carseat. Not as big of a deal in the US, as most people aren't looking to steal the baby mee.
The metal frame has springs on either side in order to provide some give on the bumps. It also includes two bungee cords to hold down the carseat. Our carseat is a tight fit. So although it instructs you to clip them around the handles I use it across as you would a lap belt installation in a car. The plastic clips coming together in the center. For what it is worth it seems very secure but I have thought about installing a seat belt on the bottom to hold the seat in place even more so then the clips.
The baby mee also provides you space under the carseat to store stuff. I keep water bottles and our locks underneath something I could not do previously. Our toddler doesn't get any more foot room with the new set up so he places his feet on the carseat still. Not a huge deal as he can't reach the floor of the arrow yet anyway.
Overall the baby mee is a helpful addition with an infant. It is rated for two months plus but I'm sure people use it pretty early on. We would have had as well if we gotten it earlier then it arrived.