I can’t speak for thoughtbot, as I’m just a subscriber, but the mentoring doesn’t seems like the right medium for full on code reviews. If your looking for someone to look through lots of code to give you tips on where to improve, the forums and office hours seem like better mediums.
The mentoring has been far more about discussing direction of all aspects of development. What should you be doing next? Is it time to dive into testing? How do you prepare for your first Rails job or moving to a new one? Yes the aim is improved code, but that should be getting done via other avenues. The mentoring, for me at least, has been far more about broader goals.
There was a post here a few months ago about the benefits of having a mentor where I posted about some of what you might expect from the program. However, there is a cost attached and if you go in with the expectation of it not being worth the money then it’s not a good choice for you. Having a mentor isn’t going to make you a great developer. Wanting to improve is going to make you great and if you think the mentor program will help then you’ll reap the benefits.
I know this doesn’t answer your questions but I thought insight from someone who’s had a mentor for 5 months and is still getting value might help justify the price difference.