Our baby slept on one of these from 2 weeks. I didn't put him on it straight away as I thought the curve looked a bit big for his head. With hindsight this was wrong and he was comfy on it straight away and slept really well.
The fact that it "opens up the airways" is a real plus and gave us peace of mind as well as being a benefit for our baby. I also noticed that where other babies his age were developing 'flat patches' on their heads, our son didn't. Due to the curve, you can't do 'feet to foot' blankets which was I wasn't sure about, but actually our baby was swaddled at first then in a sleeping bag when he was older so this wasn't a problem at all.
Once he'd outgrown his Moses basket, we put our son on a SleepCurve mattress in his cot and he continued to sleep really well (through the night from 10 weeks), so he must be pretty happy with his mattress. Would and have recommended SleepCurve to my friends.