It all depends on your area, who you kill, and how smart you are.
I've heard from local LE that roughly 1/3rd of murders are solved in my county. The forensic techniques you see on Court TV and CSI are rarely applied in a lot of murder cases unless the victim is someone high up (sad, but that's how things work).
I'd imagine it'd be pretty easy to get away with a random stabbing if there are no witnesses, no cameras, and the place is frequently traveled and if you don't leave any solid evidence behind (such as a knife with your fingerprints on it).