The playtest version of the Slayer came out late last year and the final version is coming this summer. The Slayer is supposed to be a hybrid class of a Ranger and a Rogue and borrows many of the best features from both.
Now I will do my best from going into why I don't like the Rogue class but many, many people like the idea of a rogue so if there was another class out there that do the same sorts of things but only better then the world will be a happier place for all.
Few classes can do melee combat well. In later levels there are lots of heavy hitters and a good melee combatant needs some way to take (or avoid) the hits. There is no perfect solution but as long as you have your basses covered in at least one area you should be ok.
The first is AC. Have a high enough armor class and you are almost unhittable. The PC Sergent Fenton in my current campaign is a good example of this. Most sessions go by without him getting hit at all. Even his touch AC is really good.
The second is DR. It doesn't matter if you get hit with every swing if they have to subtract 10 or 15 points of damage from each blow.
The third is avoidance. There are a few options here. You can use Mirror Image, Displacement, if available pre-nerf Crane Wing, or Duelist's Parry.
Since the Slayer (like the Rogue and Ranger) can't really achieve any of these three objective ( a dex based sword and board Ranger might be able to pull off AC), they are best suited for ranged combat.
So how good are Slayers at range combat? Let's look at their derivative classes to get a starting point. The rogue is pretty bad. Sure she can can do some serious damage when she has surprise but once she's seen those 1d6+5 arrows are a joke. The ranger does much better. In fact, I would argue its the rangers most effective style. In Pathfinder archers are crazy powerful since they can just stand at the back and launch volley after volley of death. It is a bit boring to play but hugely effective.
The good news here is that the Slayer is perhaps an even better archer than a Ranger. The Slayer will get sneak attack on each arrow during surprise but remains very effective even without it. The ranger gets a few spells that can help out but in any case they are quite comparable.
In a campaign where all you face are Drow, the Ranger is much better since your favored enemy is obvious. Any other time the slayer is really good since you just spend a standard/move/swift action studying your foes and you instantly get a bonus equal to half of the rangers (Favored Target). No limits per day, no limits on enemy type.
What else makes the Slayer awesome? You still get to throw lots of sneak attack dice. Not as many as a rogue but its still good. The better part is that you don't do trivial damage without it.
You get slayer talents, which are like Rogue talents but better. You get to cherry pick most of the best talents form the Rogue and Ninja list and get a few awesome ones of your own. You can pick Trapfinding making you the ultimate combat rogue. You can take Ranger Weapon Styles as talents. You can get Evasion as a Talent. You get full BAB so you can hit things. Favored Target really makes sure you hit things. You still get a decent number of skills and you should have decent hit points.
What sucks about the Slayer? They still have crappy saves with no easy way to fix that. They still get beat hard in melee combat but do have a few more hit points so its not quite as bad as a Rogue.
Overall I really like the Slayer class (not crazy about the name). It is a great Rogue replacement that is effective in combat without trying too hard. Bards still make better skill monkeys but not everyone likes classes with spells. Just tell your adventuring buddies that you are a rogue, do rogue stuff but be awesome at combat as a Slayer.
Let's hope the final version of the class doesn't get nerfed.