After more than a bit of time spent with the class, I really think its 3rd level spells could use a change. While Spider Poison and Poison are neat abilities for flavor and certainly fit the class, they are useless. The only time I ever bothered using either was for testing, and the results then were simply that these spells are not worth my time against any foe. The DCs are terrible, for a class that relies on two main stats, and the effects, even in the unlikely event something/someone fails the DC, are really not worth the action cycle you spent. Assassins already get Heart Ripper to simulate their ability and tendency toward murder. The limited spell selection and spell casts make it a no brainer, but for balance sakes, there should be a sacrifice when playing this PrC and casting a specific spell. Your options should dwindle every time you do, and at the moment, 3rd level spells are just four casts of Insight, or Deep Slumber for teh lulz.
I propose to switch them out for two other spells that while, not as flavorful for the class, they fit just the same. Expeditious Retreat and See Invisibility. At current, Assassins are the only ones capable of only casting Invisibility but not something that detects it. Putting this spell at the 3rd level will make sure there's a long time before someone can cast it (Level 6 Assassin if you don't have 16 INT, and if you do, Level 5 Assassin), while also improving quality of life. As for Expeditious Retreat, the name of the spell illustrates the point of it being available to the class. None of the benefits of an actual 3rd level spell like Haste, but simply a tool for an Assassin to quickly escape the scene of a failed or successful job.
In addition, fixing Arcane Spell Failure on the spells would be welcome (currently doesn't apply, hasn't happened once even when in chainmail and a tower shield for testing purposes), as well as correcting Caster Level scaling. Durations for spells remain innate, but the DCs scale. This becomes annoying, often.
Here's the end of my sort of short and sort of sweet review, thanks for reading chums.