The trouble is, lumping all cereal bars together isn't very helpful.
The ones I use are the 9-bar - made mainly of seeds. It's high in calories compared with most, but also higher in fat and protein, which makes it a lot more satisfying. I use it mostly as a meal replacement - it doesn't spike my blood sugar, I suppose because it's high in fat and has about 12g carbs.
I also like the Nak'd bars - just the ginger bread one and the pecan pie one, both of which are around 12g carbs.
Very occasionally I'll eat an Alpen light bar, which are relatively low in everything simply because there's not much to them - as a result not very satisfying but lower in calories.
I used to love those Kellogs Nutri-grain bars, which are basically cake, but since counting carbs I've stopped eating those.
Yes, they are generally highly processed (with the exception of the Nak'd bars which are just fruit and nuts) but they are carb and calorie counted, individually wrapped and portion controlled, which makes them very convenient to eat on the go.