Happy about the extra capacity but having recently flown the new Finnair J product HEL-JFK I'm not a fan of the seat.
Finnish crew were awesome, food fine, but I honestly cannot recommend the J seat especially for anyone with mobility issues.
Seated it is fine and the foot rest is very comfy.
But once converted to be able to lie flat, it takes some effort to scoot down into the footwell. The mattress topper is thin, so bed felt harder than other airlines. The seat belt is retractable, so you can't have it loose like lap belts on other airlines, meaning it is tight on hips and makes it difficult to adjust the duvet or roll over.
I was sporting a knee injury so when it came time to get up to use the loo, I found it extremely difficult to get back up to a seated position, so I could lower the gap filler and get out. There is nothing for your feet to get purchase on to push up and similarly nothing you can grip to pull yourself up - its all smooth curved surfaces no handle. After awkwardly managing to get up, I didn't dare lie down again.
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