[Do] they keep you dry when busting through soaking wet alders all day in the pouring rain.
Better than anything else, but you'll still be damp, or wet.
Also how well they will hold up when it gets dirty(does it breakdown somewhat like goretex)
A 'wetted out' outside will not allow vapor to pass. This can happen with extended abraison, or a film of dirt. A greased up inside will not allow vapor to pass - sunscreen, body oils, etc. eVent is no different from Gortex in this regard.
Will it rip/tear easily if hung up in the brush?
Devils Club can kill it. Running into the stump of a branch can tear it. But with just a little care and no surprises it will do very well. Most garments made with eVent are designed to withstand a fair amount of abuse. Pointy things, small or large, should be avoided.
In short, most garments by the industry leaders are sturdy, and eVent is the most breathable in its class.