Pros:High weight limit, infant pillow, easy to install, luxury feel
The Bottom Line: If you are willing to spend the money, the Decathlon is an excellent, luxurious seat. It is easy to install, has a great weight range, and is very durable.
I bought this car seat when my older son was four months old. He was getting to large for his infant carrier so I needed a convertible seat. I was willing to pay for quality because he hated the car. He would scream on any drive lasting more than 10 minutes.
I naturally gravitated towards Britax because they have such a great reputation for quality and ease of installation. The safest seat is the one that will be used correctly every time. If it is easy to properly install that says safety to me. The fabrics on the Britax also have much nicer feel. They feel like high quality fabrics that will last. Also the Britax has more cushioning than any of the other convertible seats that I saw. My son needed to be comfortable because he hated car rides.
The reason I went with the Decathlon over the Marathon was the removable infant pillow. I did have to pay more because of it, but when your child lengths out of the infant seat at such a young age, you don't want him lost inside his new seat. The pillow provided all the extra support he needed.
I know that there are after market products available, but car seat techs will tell you that they are not safe because they have not been tested with the seat. One just does not know how they will affect the safety of the seat. Also the seat was not designed to be used with them.
The pillow also folds over if you are going to start with a newborn. This seat is approved for children 5-65 pounds. It rear faces to 33 pounds I believe.
The seat appears very large but it took up less room than the infant seat with regard to the front seats. It does have a wide footprint making it difficult to put additional people next to it a mid-sized car. If it is an outboard position and the other back seat rider sits in the other outboard position, there is plenty of room however.
The installation is as easy as had been reported. I can install the seat with LATCH or a seat belt and have zero movement. It is rock solid and will not budge.
There are is velcro patches on the straps and side of the seat to help keep the straps out of the way while placing the child in the seat.
It has a top tether that can be used rear or forward facing. It has been challenge finding an anchor point for the top tether in the rear facing position, but cars do not seem to be designed with that in mind. It is not required to use the top tether while rear facing.
The high top slots for the harness and high weight limit. The seat will be useful for a long time.
The infant pillow support small children who are still rear facing.
It sits up high enabling children a better view. That was a sanity saver for my son and me.
It installs easily with detailed instructions in the owners manual and on the seat.
Good fabric and cushioning for child's comfort.
Velcro to hold the straps in out of the way for child placement.
What doesn't work
It is expensive. There are perfectly good seats for a lot less money.
It is large and heavy. It may not be suitable for small cars. It would be quite a pain to lug the seat through the airport.
The seat must be removed to change the harness height.
If you are looking for a high quality, easy to install, seat that has a few extras, this could easily be the seat for you.