In solo play her ability to foresee the future for relatively cheap makes her damningly powerful. Never again shall the mythos deck contain nasty surprises that you aren't ready for. Hell, sometimes it's even worth losing a turn if the card that threatens to come out is that bad. This effect is easily enough to justify her exorbitant costs, assuming you splurge for some ways to boost your income.
Now I think in multiplayer her role changes slightly. Sure knowing what's coming will help someone in the party survive, but it's a diminishing effect when you're pulling 4 encounter cards a turn. Here I think Alyssa is better served as a guardian angel for particularly time-sensitive weaknesses. Keep Zoey or Roland from pulling their signatures when the clock is running out, or keep watch over the person in your group who has a large pool of resources they're saving for something and a bit of a paranoia streak.
Finally the 'bonuses' of +1 books and a decent sanity body aren't too shabby either. Of course, both of our current Mystics are well equipped to deal with horror, but it doesn't hurt. Plus sometimes you just have to investigate.
Is she better than Skids? No, but she might certainly be worth charisma in solo, or if you're just not the Skids guy in multiplayer she's worth the open ally slot.