I used a grobag on my DD from about 2 weeks of age, but still wrapped her arms so she wouldn't flap around and wake herself up. I also tucked her under a wool blanket when it was really cold at night (like the nights I stayed at my folks' house in Harvey - brrrr!) and a lighter flannelette when it wasn't so cold (the lighter just so she felt more secure). They do say on the packet you're not supposed to need anything else, but I found she did for both warmth and security for her - she wasn't going anywhere so there was no danger for her. As for clothes underneath, she had on one of those bonds grosuits with either a short or long sleeved bodysuit underneath, depending on the weather.

I found them invaluable - getting up at 3am when it was freezing and being able to pick her up without having to really take her out of her nice cocoon of warm air - she stirred far less with feeds and got back to sleep very quickly. Definitely a nice thing for a sleep deprived mummy!