I purchased one of the Shifts because I wanted a lightweight bag for those hot hot Houston days when a full bag is too much. The bag is light and easy to carry, but has some shortcomings.
The best thing about the bag is truly it's small footprint, it is very light and due to the limitations in the number of discs it carries, you are forced to keep it light. The other thing I noticed is quality, by my measure it's a well made bag. I've not carried it enough to know how it will stand up over time yet.
The bad is numerous. My pet peeve is that it doesn't stand up well, that is, it wants to fall over side to side. The simple fact is that unless you have at least one disc in each of the four side pockets, it wants to fall or sag away from the straps and tends to then fall over. To some extent, this is due to the small footprint, but a stiffer bag would solve the problem. For example, I cut two triangles out of 1/8 inch plywood that insert inside the bag (they aren't perfect triangles, I notched them in so they wouldn't interfere with disc removal) on each side that make the bag much stiffer and prevent it from sagging and falling over.
While the pockets on the side can hold two discs each, if one pocket has two, it makes it harder to get two in the other. Not hugely, but the discs don't slide in easily. It's even worse in the central pockets, but the limit is three. If you get three discs in one pocket, the pocket next to it becomes more difficult to use. Along with this, getting the discs to lie stacked in a way that is convenient now requires effort. You have to slide them about to get that, oh lets call it cool, stacked look. If you don't work at it, they tend to all slide down or the one in the back slides down leaving the one in the front higher.
Because of the disc loading issues, I find that realistically, 12 discs works best. Yep, you can get 16 to 18 in there, but managing them takes some effort. Is this a killer, nope, just annoying for me.
I've seen a couple of comments above about using the bottom pocket for extra discs and I do this. I don't carry much in a light weight bag and the thought that I'm giving something up to carry a few extra discs doesn't bother me. I can fit my phone and wallet in there and even an extra towel with three or more discs, so I'm not to worried. I don't use the inside side pockets, stiffeners are in them, but that is where I'd put them if I had no support structures in the bag. I don't use the waist pocket carriers, but can see how they might be a boon for those who did.
The big sell. While the short comings of the bag bother me, it more than serves the need I got it for, a very lightweight easy to carry and move bag. Frankly, it looks great, and I find carrying it, even on long rounds, quite pleasant relative to a full size bag. I wouldn't get if for tournament play unless you are a minimalist, and I've seen enough guys who use six or seven discs exclusively during rounds to know they are out there. With the rainfly, you can't argue that it wouldn't work. If you have a stool that you sit your bag on you might have a problem, but you certainly will enjoy the lighter load.