In my opinion, this is the first supplement of Four Against Darkness that a player should buy, after picking up the basic game. I personally consider it essential, given how much it adds.
The process of turning your character into an Expert is well-laid out, fairly simple and definitely feels significant. And, of course, as the heroes grow in power, the challenges become more formidable.
The Expert Skills add a lot of customization to your character, fleshing out their style and abilities. I actually have a house rule that's worked very well in my own play: every character gains an Expert Skill at first level BUT, the tradeoff is that Bosses and Weird Monsters do not drop a level at half health. Certain monsters are more of a challenge but the heroes can leverage their skill throughout the campaign.
I liked most of the Expert Spells, with the minor exception of Lifeforce Control, which seems underwhelming in comparison to the others.
I'm also a fan of the rules for hiring Professionals and Hirelings- it's a new and useful way to spend your treasure between adventures and can be very useful to the party.
All in all, top marks!