Finally a thread where I can post this fully..
Assassins are of "dubious" worth IMO.
The concept itself is cool, but the execution of the class is poor.
Conceptually its all about that first attack.. and perhaps the 2nd or even 3rd if magically enhanced with Hast etc. and Improved Initiative, and Blooded or Thug (also for the improvement in initiative) - and at very high levels perhaps Epic Two-Weapon Fighting. It is NOT an "attack of opportunity" character - at least not anymore than any other sneak attack class.
Whats good about them:
1. Death Attack's paralysis feature - if "hit" effectively making all subsequent attacks sneak attack damage IF the opponent can be sneak attacked.
2. HiPS at 8th level.
3. Its a prestige class, so no XP point penalty possible.
4. Its a prestige class WITH the potential for HiPS AND Sneak Attack damage.
Whats bad about them:
1. Death Attack's paralysis feature is a *fortitude* saved based on your *Assassin* level and your Intelligence bonus. The computation is 10 + Assassin level + Intelligence bonus.
a. first note that it is a *fortitude* save, while this bodes well against spell casters who usually have a pitiful fortitude, its bad against strong melee-type opponents - opponents which actually compromise more than 95% of the game! Additionally consider which opponent-type you really want it to work on - a spell caster who has fewer hit-points or a melee-type who has a lot?
b. second - the save is level dependent, of course so are most opponents. Roughly speaking this means that it never really gets better even as you climb in level, and of course as a result that means its not very good when you do start accumulating levels. Its also a prestige class and you can't start accumulating levels really early when it would provide its greatest advantage against weaker *fortitude* opponents. (note that character level 6 is the *earliest* possible time you could select this class, at this level you may well have 2 or 3 character levels where the paralysis seems to work better, but it won't be for very long and that doesn't include the fact that your ability to become Stealth is usually weaker at this stage of development.)
Then follow-up with the now obvious detriment - that you need as ***many*** class levels as possible to really make Death Attack's paralysis to work for your character.
c. third - the bonus is based on an Intelligence modifier! Who in He!! makes an Intelligence-based rogue-type character? Most rogue-types are struggling just to HIT their opponents because of the medium BAB. So they will usually take Weapon Finesse and up Dexterity to its max-potential at the exclusion of all other attributes. The reality is: there isn't any real room for gains in Intelligence - even attribute enhancing items are generally eschewed in favor of Dexterity and Constitution enhancing items.
..the net result of all of this is that EVEN IF YOU:
Have 10 levels of Assassin (plus your base 10), AND
Have an enhanced Intelligence netting 20.
That leaves you with a *TOP* save value of 25.
Then factor in that the save can still fail on a roll of a 20.
IMO this is not a feature you can count-on at *any* level - and thats kinda' the point of an Assassin! I.E. that you can count on at least most of the time making your Paralysis feature work against that one opponent. The real "work" should be getting into to make the attack in the first place and then getting out alive - and that problem is still *very* much in place.
(..yes, your opponent can also role a "1", and if they likely don't have epic resilience then they will fail their save and be paralyzed, but again: it certainly isn't something you can count on.)
..and if that wasn't bad enough.
2. Death Attack's paralysis feature is ALSO dependent on an opponent that can be sneak attacked! If its immune to criticals or sneak attack then forget it working. And NO, Epic Precision does NOT make an opponent Death Attack-able. Epic Precision ONLY passes through HALF your sneak attack damage, your Death Attack's paralysis feature does NOT work with it.
..worse still.
3. Death Attack's Paralysis only has the chance of working if your opponent is not in combat mode. Practically speaking this means that you have that one chance to get a Paralysis hit. It is truly what you would think of as a Sneak Attack. The problem of course is that its use is severely limited, but frankly not in a manner you would ordinarily take exception to (..excepting all the other cr@ppy limitations that are NOT apparent).
..but wait, there is more!
4. Death Attack's Paralysis pretty much works *only* with a melee attack. Supposedly you can attack with a ranged weapon with in a 30 foot range of the opponent, but I haven't gotten that to work. Want a sniper character? Look else where.
Ok then, enough about Death Attack's worthless nature..
5. HiPS requires an 8 level expenditure. Umm, a Shadow Dancer only requires *1* level to get HiPS - you do the math.

Now it is true that a Shadow Dancer ALSO requires two feats: Dodge and Mobility.. BUT almost any one crafting a HiPS *usable* character will require the feat Spring Attack (..and yup, that requires Dodge and Mobility). A Shadow Dancer also requires 2 more points in Hide than an Assassin, but again - for HiPS to have any real chance of working well it *needs* a high stealth character.
..
Note that NONE of the above is to say that the class is worthless, but rather that in most instances a standard Rogue with one level of Shadow Dancer is at *least* as good.
(..note that a higher level Rogue has SEVERAL real advantages over an Assassin. That and perhaps the use of a Warlock may be a much better - alternatives that I might delve into next here..)