I bought muslins to use for the first few weeks. TBH, I never got the hang of them. Maybe I'm just rubbish at flat nappies, but I couldn't get them to fit well on Iris. We ended up using shaped and pocket nappies straight away. Like Hanna, I found BG to be great from newborn; excellent fit and never, ever leaked (even explosive BF poo!).
just to add about muslins, the good quality muslins are double-woven whereas i suspect the cheap muslins are only single thickness and therefore perhaps not very absorbant. the best price i've seen for muslins recently is bebeco, they have Imse Vimse unbleached 80x80cm muslins (double-woven) for 4.99 pounds for a packet of 4, or 3 packets for 12.75.