I'm torn on them! I'd probably summarise by saying that it's best to do without if you can though...
We used one with DC1, but it wasn't a magical sleeping cure for us, like some people claim. Certainly didn't help during the Fourth Trimester, but when baby got bigger and actually started sleeping in the bedside crib (rather than in bed next to me), I do think the feeling of snugness/security was beneficial then. But then we tried to take it away in the big cot and sleep was awful, so ended up shelling out for the bloody toddler one in desperation.
I have to say I probably hated them more than I loved them! Replacing of the covers was a bloody nightmare and the mattress pad NEVER went back in perfectly flat (which is how it comes when new) which pissed me off for the price of the thing. That's despite following the washing instructions carefullyand washing on a lower than recommend temp to avoid shrinkage.
I'm due another baby imminently and torn as to whether to wash the smaller one ready to use, or just dig it out once baby arrives if we feel the need. I would rather follow the safe sleeping guidelines, if baby will tolerate being put down at all, or just bedshare safely in the first instance I think. But sleepyhead might be handy for using downstairs as somewhere safe to put baby when toddler needs me... and can be used for tummy time, etc.
I have to say I watched DC1 (obsessively) in both sleepyheads and their face was never anywhere near the bumper bit when I was supervising sleep (obviously can't speak of the times I was asleep too) - and it was definitely something that concerned me. DC1 was a very still baby though, not a figety sleeper, happy on back, didn't roll in sleep for a very long time. I'd be less confident using it with a very wriggly sleeper.