Rating: 5 / 5
(The MLD Zip is the precursor to the current 2009 Exodus - changes aren't huge, and all appear to be improvements).
I've used the Zip for perhaps 1000km over several months - it's an easy 5.
I switched to the Zip from the Golite Jam2 because I wanted something lighter & a bit more flexible. Overall it's a similar type of bag to the Jam2 - frameless, a large rear pocket and two smaller side pockets. I was expecting it to be slightly smaller in the main compartment than the Jam2, but I think it's actually slightly bigger. Although I liked my Jam2, I've found the Zip a much better bag in many ways: it's lighter, more comfortable, and has many more features.
The size is good: enough room for a 5 day solo winter trip (down gear, tent), but light enough that it's barely worth trying to find a lighter bag for a summer overnighter. I roll a torso length NightLight pad into a cylinder inside the pack and pack my gear inside this (hat tip to Andrew Skurka's site for that). I think my max load was 22lbs for any real length of time, which was comfortable all day - obviously this is a personal thing, but I think the bag would still have been comfortable with a higher load.
The rear mesh pocket is massively useful, much more so than the zipped rear pocket on the Jam (I can fit more, I can see what's in there, and I can get stuff in & out much easier). Because of this I rarely need to open the main pack pocket during the day, even with big weather changes. The mesh might seem a weak point, but it's not - it's darn tough. I don't bother with compression straps over the top of the mesh pocket, but you can put them on if you want. Nothing has ever fallen out of it (despite much falling by me).
The attachment options right over the bag (shoulder straps, hip wings, in & outside of the rear mesh pocket, up and down the back) make the bag incredibly versatile - you can tie on hip/chest pockets, compression bungees (including over or under the rear pocket), pole hoops, clip on a thermometer/stopwatch, camera. Or nothing at all if you'd rather. Very very cool.
I consider it pretty much bulletproof - even with the silnylon back (versus the full dyneema Jam2 & current MLD packs) the pack shows no signs of wear, even after bashing through many rough trails.
One minor niggle which has been addressed in the newer version: the top strap is a bit thin and can sometimes fold over in it's clip.
Right now the only bag I'd consider replacing this with would be the newer version.