Considering I don't have a 3.5e MM with me, does the Doppleganger have Move Silently/Hide on its Racial HD Skill list? If not, assuming my memory of the assassin's requisites are correct, the doppleganger would not be able to meet its requisites til after ECL 14. (4 HD that grant 2 skill points, 6 levels that grant 6 skill points, and 4 ECL) Which means, regardless if it has +1 more bonus from its Int compared to the other two assassins, its DC will be lesser. Please correct me if I'm wrong here.
You are right about the doppleganger's skill set. I would argue, however, that the assasin as presented in the DMG is self-limiting itself to humanoids in general in terms of its prerequisites. First, anyone
could be trained as an assasin, the game and its rules limit this to characters who have sufficient ability so that the average guild would accept them. Picture said guild when a rogue (as in, not with a bunch of its kind) doppleganger comes on the scene. "I can assume any shape I wish, get in wherever I want." At that point the guildmaster does not ask "Yes, but can you sneak around?" The possibilities are equal if not greater for a doppleganger to penetrate anywhere an assasin (or rogue) could with Hide/Move Silently. It would be nice if the SS had given alternate prerequisites so that monsters need only meet a BAB restriction and have some "special quality or attack" to get into certain prcs. For instance, a shadow would make a perfect shadowdancer
)) and something like the Xill (a natural planeswalker) would be a perfect Horizon Walker.
As I said though, you are right. A doppleganger does not qualify for the assasin prc. However, in some ways, the perfect doppleganger assasin can be made just taking rogue and fighter levels, or even by taking shadowdancer levels. While the death attack is what gives an assasin its prestige (yuk yuk) as Artemis Entreri has proven, being a good assasin just means you kill people.
As I am mostly a player using 3e, where a Hat of Disguise costs only 2,000 gold (a fact I have not yet checked in 3.5e)-and while more limited-it produces a skill bonus rivaled to the Doppelganger's natural changing ability at a cheap cost. And while Disguise and Bluff are improtant, there are other skills as well which the other two would be more proficient with. The Doppleganger may know what's behind the door, but the drow rogue would have a better chance of picking the lock on it, for instance (his higher max skill ranks due to HD would still be better than a +1 bonus the other two may have from higher stat bonuses).
Different ways to do the same thing. While a doppleganger may not be the thief that can guide you down a trapped corridor detecting, disabling (and possibly resetting) the traps along the way, the doppleganger may have been able to avoid the corridor entirely through different channels. Not in your typical abandoned ruins/dungeon, but in social interactions he always has the upper-hand.
Furthermore, a Hat of Disguise is little more than an insta-disguise kit. You can't appear more than 1 ft. different height-wise, it doesn't alter the tactile (nor even the perceived tactile) properties of your form. You touch a doppleganger that looks like a knight in shining armor, you touch armor. A rogue with a hat of disguise, you touch leather (unless the rogue had time to preprare said disguise by putting on some full-plate). It is also dispelled by touch, or interacting with the said disguised person. While it may be useful to get you through the city gates, it won't get you in the castle, if you know what I mean.
The will save may be a weakness, yes, but unlike the doppleganger, the half-fiend is an Outsider, and thus has natural resistance to Charm Person and Dominate Person, the latter spell one of the more feared powers available to wizards this level. And not to mention, as this is for ECL 9, 25% of all spells on average will bounce of the fiend anyways, not just the ones reliant on Will saves (and 40% for the Drow, who has a further +2 bonus to Will saves vs. spells).
Close, but no cigar. First, the only one getting affected by things like Charm Person or Dominate Person is the Drow, as the doppleganger is a monstrous humanoid. Of course, even if it weren't, the doppleganger is immune to charms, so charm monster (which is effective against both) won't bother the doppleganger. Furthermore, most of the time spellcasters are "choosy" when it comes to picking their targets, a doppleganger will be mistaken for the "right" person to target far more than drow or half-fiend.
Getting to the save bonuses, and this is really an easy fight. A similarly made character has at least a +6 (with inherent immunities) over the drow and half-fiend when it comes to Will Saves. Those Monstrous Humanoid levels aren't good for much, but they do have Good BAB and Good Reflex and Will saves. While the drow are naturally spell resilient (SR 11 + Char level) they are always 2 levels behind (which is where you got 40%). The half-fiend is in the same boat, but 4 levels behind (and SR 10 + Char level) which is where you got 25%. However, these are the base chances, any bbeg will be at least 1 or 2 levels higher, and if it is a dedicated spellcaster may even have Spell Penetration and Greater Spell Penetration, or some bonus to caster levels.
Look at most of the upper level Will-save spells: Charm Monster, Feeblemind, Hold Monster, Nightmare, Mass Suggestion, Insanity, Harm, etc. These spells are significantly easier for a doppleganger to make due to his higher will save. You bring up evasion, but the doppleganger is just as likely to make a save as either of the other 2 and most spells offer reflex saves for half damage. Half-damage still hurts (especially when you are 4 levels behind the party), but the doppleganger is the most hardy.
Overall, it just comes down to the type of campaign. In a typical campaign, I think it is a decent option for a mid-level character who wants something different in their session. In a heavy-combat campaign, the doppleganger is under the curve (as are any characters who suffer an ECL, as others have pointed out). In a campaign more focused on social events, or even just a combat-lite group (one who solves problems more than starts fights) the doppleganger is a steal being allowed to come in the party period.
On a side note, I'm a little sad they didn't include a Greater Doppleganger. They were quite a bit more difficult than the regular ones in Baldur's Gate, but I think the doppleganger has the room to add some cool abilities. At the very least I think the regular doppleganger deserves a bonus feat - Persuasuive, Negotiator, or Deceitful would all be decent choices, imo.
Technik
