Back with "Singular Destiny" I thought that the Typhon Pact would make an interesting enemy, but since then I haven't really found the Pact to be that enemy that they were setup to be. I've read up to Brinkmanship, but right now I've found the Pact is more of a mouse giving off the roar of a tiger and doesn't really seem to be as big of a threat, or even be in the same class of enemy to the Federation as the Cardassians/Dominion or the Borg. The Pact seems to be more like those two races from "The Outrageous Okona", and my reaction tends to be like Riker's in that episode when he says that the regs do call for shields against pea shooters.
I've only read Cold Equations
and Zero Sum Game
so the Typhon Pact is coming off as pretty formidable so far. Admittedly, the Breen are coming off a bit like Starscream so far. I.e. the guy who does more damage to you than your enemies.