Good posts, all!
I think part of the problem, at least from my point of view, is that the assassin's
pièce de résistance is his Death Attack. Other than that - speaking strictly from a mechanical standpoint - he kind of falls flat. A second-hand rogue as some people have alluded to (or directly said

). Case Target is a nice ability, and can be helpful, if utilized properly. But the Death Attack is what makes the addaddin stand out, in my opinion. Only the monk gets an "attack and you die" type of maneuver, and assassin's get it from the beginning.
My issue (once again, this is opinion) is the host of creatures that an assassin will face. Assuming they will be part of a normal group of adventurers doing "normal" adventuring stuff, of course, and not a specific intrigue/city/political type game, most of the foes (and I haven't done a physical count, but I'd wager 75% or more) from the books (M&T, M&ToA, CM) have physical saves. This could make the assassin's defining ability difficult to pull off. I know I wouldn't want it too easy, of course, but it's definitely an uphill battle as-is. Compare that to the monk, who doesn't need multiple rounds to study his victim (but has to wait until 10th level and can only do it once/week) who gets lots of other interesting abilities that don't make him look like a weaker version of another class.
Now they get some nifty things from the CKG levels, but that's much later in their careers.