There is nothing wrong with The Good Liar, and there are a couple of identifiable good things that will at the least please a certain segment of the film-going audience. If a movie starring two venerable English actors showing what they can do in a plot that features a twist or two sounds like something you'd like to see, you're probably right. (Of course, I could be describing the Olivier/Caine Sleuth.) Russell Tovey acquits himself well along with the Dame and the Sir. And Jim "Carson" Carter of Downton Abbey fame has a sizable role. Like I say, nothing wrong ... The Good Liar is harmless.
Nonetheless, there are no overwhelming reasons to see The Good Liar. If you need a Helen Mirren fix, The Long Good Friday or Gosford Park are better. If you want to see Ian McKellen, Gods and Monsters is a good one, and it is also directed by Bill Condon. In fact, if you only take one thing away from this post, it is that you should see Gods and Monsters, which also features the criminally underrated Brendan Fraser.