TL;DR: GSM security is a joke. LTE is okay, except for two critical issues: One, an attacker can jam LTE and cause a downgrade to GSM. Two, it doesn't offer forward secrecy, so an attacker can record your traffic, obtain the private key from your carrier, and decrypt it. It's a reasonable assumption that NSA and your local sigint agency routinely make copies of your carrier's key database.
http://blog.cryptographyengineering.com/2013/05/a-few-though...
TL;DR: GSM security is a joke. LTE is okay, except for two critical issues: One, an attacker can jam LTE and cause a downgrade to GSM. Two, it doesn't offer forward secrecy, so an attacker can record your traffic, obtain the private key from your carrier, and decrypt it. It's a reasonable assumption that NSA and your local sigint agency routinely make copies of your carrier's key database.
Edit: Reword last sentence.