Given how good GR's stuff tends to be, I was surprised to find the Assassin's Handbook underwhelming. The central class feature -- the killing blow -- is very easy to break (save DC is based on damage dealt, and it's basically a coup-de-grace attack), and is distinctly overpowered. The feats and PrCs are somewhat bland, as are most of the spells; the equipment, poisons and magic items are all pretty neat. The art is generally quite good.
Over half of the book is taken up by detailed write-ups of two assassin organizations, the Sirat and the Vultur. These are OK, and fairly modular, but I would rather have had more solid rules material, extra crunchy bits and more general info than this much background material.
In terms of wanting a non-magical assassin class, this one doesn't fit the bill: it gets a slow spell progression, much like the DMG version.
Overall, I'd say steer clear. YMMV.
Edit: it just occurred to me that you might want to consider the ninja class from Rokugan. Although it's not an assassin class, it does combine elements of rogue and fighter with an emphasis on speed, stealth, and sneak attacks. It's also a great, well-balanced class.