Official guidance will of course tell you no. However we used a sleepy head for 3/4 months and it was fine. DD was v small and cot felt too big for her, so she wouldn't settle. They are ruinously expensive though. The only thing is they are quite small, they are meant to last until 9mos but if your baby is big it won't last as long (you can get a bigger one).
What are your reasons for wanting the sleepyhead?