Though I haven't used them in a while I own some prismacolors. Prismas are great, but only if you know how to use them. As previously mentioned by my fellow gaians the lead breaks pretty easily so you can't push to hard. THe best technique is a gradual build up, it's time consuming but can give beautiful results. Once I get around to taking pictures of some of my old prisma work I'll post them.
As for graphite pencils (my very favorite medium) I usually start with a 3h or 4h to apply a base and gradually work my way up to about a 3b. The h pencils have a harder lead, which gives lighter line work and isn't as greasy, the higher the b the greaser/softer the graphite and so you'll want to make a gradual build up with them. This
Drawing gives some idea of how I work.