As in, when I first saw this, I thought "Duh, why hasn't anyone made this before?"
I love this product, and use it in every game I run. I use the original version, which I would give 4 stars because of two minor complaints (some wasted space graphics-wise, and a little flimsiness in the board (which I solved by sticking the pad to the heavy cardboard it was packaged with)).
However, I can see that the new updated version makes better use of space, and also has a more solid sturdy construction, so both of my minor complaints have been addressed.
This product has made my combats so much more organized, and is useful in out-of-combat situations as well when notes or durations need to be tracked. I've been using this for over a year, and would hate to run a combat without it. It's that helpful.
At first glance, you can see the work that went into this PDF - it's clean, easily understood, good-looking, well-edited, and the illustrations of the "iconic" shadow assassin are eye-pleasing and evocative.
As for the mechanics - I made a 5th level elven shadow assassin, and played him in two sessions of the CotCT campaign I'm in.
I had a lot of fun making the character - so many of the class features are pick and choose, similar to the rogue's talents. You can easily customize the feel of the class via ability selection. There are some abilities that all shadow assassins get that focus on 1-on-1 fights, and the pick-and-choose abilities are based around things like mobility, stealth, minor spells, or combat.
I decided to give my PC an ability that kept him always armed, as well as one that helped him hinder spellcasters. My idea was that he'd be a scout and a support combatant. It worked out fairly well. In a fight with a "boss" spellcaster, I was able to provide consistent damage and spell disruption, and good mobility meant I could pursue the BBEG and chase him into a corner. In that kind of fight, I was right up with the tanks in combat contribution.
In non-combat encounters, I was still able to participate. I didn't have rogue trap skills or ranger tracking skills, but decent skill points and shadow-related bonuses let me do some good scouting.
The class does have a 1-on-1 combat focus, but it lacks an "instant kill" kind of ability (no death attack or sneak attack) which may disappoint some folks. It's also very dependent on context - if your GM isn't willing to provide opportunities for your PC to shine, you won't be as effective. Basically, this means including areas of darkness/shadow and at least one solo or boss enemy to fight - not that tall an order for a typical adventure.
tl;dr - A very versatile stealth-based class. An excellent supporting class. I will be playing this one again.