But then what if Google pulls the rug out from under apps that rely on it and what if knowledgeable users like you don't like the idea of a third party having access to their second factor?
I'm starting to think that unless you're willing to build your own authenticator apps for multiple mobile OSes SMS-only is the best way to go.
As has been pointed out, it's open source (specifically, Apache 2.0). So, fork the code, if necessary find&replace any google trademarks, and republish as a dedicated authenticator for your own app. Or use one of the existing apps which have forked off gauthenticator, e.g. https://github.com/kaie/otp-authenticator-android .
 Except for some bits specific to gmail's 2-factor workflow added after v2.21
 git clone https://code.google.com/p/google-authenticator/
Or I can implement or build from source a TFA app I trust and use that.
I really hate sites that support TFA and don't support authentication apps as I have very poor phone service at both my home and place of work and hence SMS is a frustrating experience for me.
IMO a shared-secret OTP app is certainly not unbreakable but is more secure than SMS.
SMS is known to be easily subpoenaed and universally stored while believing in a widespread OTP app trojan-horse requires some form of tinfoil-hattery. Both are still orders of magnitude more secure than single-factor authentication anyway and hence I believe both should be included in a reasonable 2-factor authentication solution.
Personally I can't adopt an SMS-only 2-factor solution due to service issues anyway.