The change mats in the nappy bags generally aren't big enough. They'll do the job when bub is a newborn, but that's about it.
Things to look for in a nappy bag:
* compartments. Make sure there are heaps of compartments as these will come in handy when bub is onto solids. You need to be able to store away food, milk/water bottles, wipes, nappies, spare set of clothes (a must in a nappy bag), washers.
* Somewhere to store hot/cold bottles. This is really handy if you have EBM/formula/milk and need to keep it cold.
* wide straps. Much easier to carry when they have wide straps.
But take into account the size of your pram also. It's handy if the nappy bag fits in the pram, that way you don't need to carry it around.