eVent rain gear?!?

[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.

I hat to say it but have you seen those long rubberized nylon rain jackets that you can get army surplus?
I have not used eVent, but having used Gore XCR and Proshell, if I was bushwacking everyday through wet alasken bush – that is what I would use to keep dry. In fact that is what I use for canoue tripping in northern Ontario – has some simaluaraties to your trip.
If you get eVent (really) dirty it needs to be washed before it is waterproof again.

I think you might find that you have to go a little heavier in some places so that you dont wear out your gear in a month long expadition like that.

"If you get eVent (really) dirty it needs to be washed before it is waterproof again."

It will remain waterproof, but not breathable, and all that sweat will feels like rain.

Before I do I was wondering how well they keep you dry when busting through soaking wet alders all day in the pouring rain.

eVent is pretty hardcore waterproof.

Will it rip/tear easily if hung up in the brush?

The thin stuff used by Montane in their Quickfire jacket for example will not be very abrasion resistant. I take good care of my jackets but there are some minor places where it starts to pill. It's not the most robust fabric.