It depends on what you compare to, what you have to protect and what your users can be billed and trained to use.
2FA with mobile phone is prone to phone theft, phone malware, phone operator's SIM replacement (mis-)procedures, mobile net vulnerabilities and so on.
It is, however, WAY better than 1FA of, say, user password. So it makes a good step at securing a great number of things. The attacker has to steal not only a password, but also attack your phone in some way or another. An attack against a phone is either easily noticed (a phone missing or not working) or complex.
My bank offers (along with a better options) SMS 2FA for a limited functionallity of their internet banking. They have to. A lot of their customers cannot be bothered to use something more complex and if you force them they will just find another bank.