I know, this class has been brought up before, and it's possible the staff are working on it now. That's why this is more of a "compare and contrast" thread with a focus on looking at what the Assassin prc obtains in his time in the field.
So, first off, we should compare assassins to their more common counterpart, and the thing that most people would choose instead of assassin if looking for a bonus damage cross class: The rogue.
At 10th level as a rogue you have:
+5d6 sneak attack
Uncanny Dodge II
And a choice of one of the following:
* crippling strike
* defensive roll
* improved evasion
* skill mastery
* slippery mind
Depending on your build, you will have 80 + 10 * int modifier skill points from this class alone.
At 10th level as an assassin you have:
+5d6 Death Attack
No dexterity check when applying poison
Uncanny Dodge III
+5 saves vs Poison
Improved Invis with a bonus to concealment 1/day
Ghostly Visage 1/day
A weakened Divine Power 1/day
2 extra acid damage
Depending on your build, you will have 40 + 10 * int modifier skill points.
So, on the surface it looks like Assassins have a ton more bonuses than their common counterpart. However, this may not be entirely accurate.
Uncanny Dodge only provides an extra +1 to reflex saves against traps over rogues.
+5 saves vs poison are not bad in a limited number of situations, but in those cases your fort save as a sneaker is probably going to be low and poisons have high DC's.
The Improved Invis is a one use per day that gives an extra 1% concealment for every two points of int modifier. So to even get an extra 5% concealment, you need an intelligence of 30.
The ghostly visage receives no bonuses until epic levels, at which point it becomes ethereal visage once per day.
The darkness would do 2d10 + int modifier damage with a fort save of 28 + int modifier against being blinded.
The weakened Divine Power raises strength to 18, gives +3 AB and raises attacks to four per round (if you have four already, this is essentially useless)
2 extra acid damage... I guess it sort of adds up? Maybe?
The issue I see is that any utility the assassin might gain through her very small selection of once per day spells (and in one case, 3/day) is negated by the rogues extra 40 skill points. Assassins actually need to consider where they're going to be putting their points and to even compare with rogues, that means tumble and UMD are a must. Almost naturally, you're going to be dumping your other skill points into hide and move silently, which leaves you with whatever your intelligence modifier brings to the table. At this point, rogues have an extra 40 skill points (or four extra skills) they can max out before even taking intelligence modifiers into account.
Then of course, the assassins abilities themselves. While they are supernatural, and as such cannot be dispelled (or so I've heard), they're not exactly something you're going to be chomping at the bit for. Improved Invis once per day is all well and good, but I can buy two or three of that just from roaming the inner fort areas for an hour, and the extra concealment is a joke. To put it in d20 terms, regular improved invis gives you 1-10 on a die roll to avoid the attack. In order to increase that to 1-11, you need an intelligence of 30.
Ghostly visage becomes a once per day 5th level spell in EPIC levels.
The darkness is quite nice, I don't have much against it, though for it to truly start being effective you need more assassin levels. And the divine power, well... Most classes will have three attacks by this point, meaning they get an extra attack and 3 AB. Since divine power doesn't stack with other strength buffs, unless you're a ranged assassin who's strength is sitting at a base of 10, probably getting up to around 16 with gear and bulls strength, then you're not even going to notice anything but possibly the extra attack and a little more AB. For ranged assassins, you'll probably have the fourth attack as a base and will only receive one extra point of damage out of the strength boost IF you have a bow with +4 mighty on it.
Now, as you progress further in levels, the gap widens a great deal. By the time you have 20 levels in rogue, you have gained:
+5d6 sneak attack for a total of +10d6
3 more bonus feats from the list provided above, and
Uncanny Dodge VI (which again, isn't that big a deal)
Get 20 levels of assassin by contrast, and you'll have gained:
+5d6 death attack for a total of +10d6
+10 to saves against poison
2 epic level bonus feats (remember, you have to be in epic levels for this)
1 extra acid damage for a total of 3
And your epic feat list is about half of what rogues have available, and the majority are quite pathetic, saying nothing of the fact that the decent ones require 10 rogue or 9 monk levels (self concealment). Oh and improved combat casting. What a wonderfully useful epic feat choice that is for an assassin
Should you end up taking 30 levels in assassin, you get:
15d6 death attack total
+15 saves versus poison total
5 epic level bonus feats total
4 extra acid damage total
Now, a lot of people wouldn't say that the spells are the defining ability of Assassin's, but that the paralysis on a death attack is. To some extent I agree, it is a defining characteristic of the class and should make it quite useful. However, let's have a look at the DC's:
The formula for working it out is 10 + assassin level + int modifier. Realistically, an assassin is not going to have a ridiculously high int modifier. For this example I'll set it relatively high at 5.
So at level 30, it's a DC 45 fortitude save.
At level 20, 35.
At level 10, 25.
And it only applies to the first attack, on the assumption that the target is currently not engaged in combat. It is also, unfortunately, hardcoded so there is nothing the staff can do about it.
The one thing I think that really jumps out at me is the emphasis on intelligence for assassins. I realise they're supposed to be smart people, but having most of their abilities (even after changes) being worked off their intelligence modifier seems counter intuitive. It essentially pigeonholes assassins into being best used by intelligence based rogues, and we all know how many of them are out there when they get 80 skill points at level 10 with an intelligence of 10.
So how do we balance the assassin a bit more fairly? To be honest, I don't know myself. Some people may argue that the assassin is already fairly balanced, but I wholeheartedly disagree.
One suggestion that has been turned over in my mind a few times is the possibility of assassin specific poisons that are unlocked as the character obtains more assassin levels. Regular weapon coatings are fine, but these poisons would be an unlimited use per day that could be chosen to drain a stat, apply a negative effect or deal extra damage over time. Higher level poisons might have two or even three effects and the DC's on them would increase greatly as the assassin gains more assassin levels. For an example, a character with 20 assassin levels might be able to choose to apply a poison that does 1d6 constitution damage and silence, and an extra 1d4 constitution damage after 1 minute, which has a DC of 15 + Assassin levels * 1.5 + Dexterity Modifier.