Yeah, part of me wants to get one too. I like my Sim a lot, but some days, it can admittedly be a bit much to carry around. If you follow the links in there, you can see that I tried out my friend's Core for a while, and really liked it. I was actually on the verge of getting one, but then the next day Chris announced the release of the Simian.
The Core I was using was actually a prototype, so there may be some subtle changes now, but there were a lot of things I liked about it. It rode really well, the weight was nicely balanced. I liked the middle pocket, not just for weight distribution, but also as a nice divider, it's a simple way to keep discs organized and kept track of. I also liked that the middle pocket has an internal pocket in it. And as people mentioned, the thing was bombproof...the Line-X is a huge selling point for me, and was the inspiration for my Grizzly Grip on my Sim. And yeah, I only used it a couple times, but the rain fly worked well.
The only real drawbacks for me were that the storage capacity was limited, i.e. you might be able to stuff a hoodie in there, but it'd be tough, and that the bag felt heavy after a while. I think the weight was primarily due to the Line-X, but I think they put it on extra thick on the proto version. I'm not 100% positive, but I think they don't put it on as heavy on the production models.
Oh yeah, I'm a big fan of the custom color options too. That was one part of the GB offerings I was really sad to see go away.