"Mediocre" is probably the most accurate way to describe the average quality of (English) voice acting in modern point & click adventure games. You may get a couple of good performances, but I've yet to play an adventure made in the last decade that hasn't bored me with dull or poorly-delivered voice acting from its non-playable characters. It's usually a budgetary issue, which is a shame, and it doesn't always ruin the game. In fact, if the game itself is really good then bad voice acting can be overlooked to a certain degree. I'd still rather good acting, though
There are exceptions of course, but most of them are older examples -- Broken Sword, Snatcher -- or aren't point & clickers at all, like LA Noire, which is by and large a modern adventure game.
Also, Telltale's games seem to fair better in the VO department, but I've played very little of them.
