we've been using grobags since DS was about 12 weeks old & love them. as mamapan says though - we've had 2 in the 6-18 months size to have 2 different tog ratings (a 1.0 & a 2.5).

in regards to overall cost - as you say, they sleep in them every night & we use also them for DS' 2 naps each day so imo they've been worth the money