Overall, I think it is a good product. The Mentalist is an easy-to-use psionicist class that looks more like a magic-users (but with its own flavor) than any of the complicated psionic systems that were created for AD&D 1e and 2e back in the days. It looks more like a 3e psion, but simpler and thus easier to use. Myself, I would be willing to play a Mentalist in a OSRIC (AD&D 1e) game, which says a lot about it. I really think this class is a great addition to OSRIC, maybe even greater than OSRIC Unearthed (also by Vigilance) as far as the willingness to play one of its class is considered.
On the bad side (so it only gets 4 stars rather than 5), I think that the graphic layout could have been better, at least it should have been identical to that of OSRIC Unearthed for those who want to print and bind together. (It would even greater if all those products had the exact same format and graphic layout as the OSRIC core rulebook, but anyway.) Also, some of the psychic powers seemed of little interest for a Mentalist player (especially those who transfer the mentalist's own hit points, or help him in melee combat, since the class is the same as a magic-user when it comes to combat). I have seen a typo about calling a power a spell somewhere.