Hydrapak's Softflask makes plenty of sense in concept. The 5oz or 8oz size lets you pack the equivalent of several gel packets into one easily accessible block, the integrated bite valve means you don't have to clumsily try to rip anything open while bouncing along the trail, and the soft, collapsible material lets you drain every last bit of energy out of the thing.
The only problem is it doesn't work all that well, at least not without some extra preparation first. The fundamental flaw is Hydrapak's bite valve, which works very well for watery fluids but simply doesn't have enough throughput to cope with more viscous gels, especially if it's cold outside.
Bite, squeeze and suck until you're blue in the face (or hypoglycemic) and the Softflask's contents still barely dribble out. The small opening also makes it a bit messy to load as most gel packet tops won't easily fit inside, and it's tough to squeeze out the last remaining bits of goo.
On the plus side, you can get gel out of the thing if you're willing to water down the contents (reducing the energy density of the gel in the process), it's surprisingly easy to clean and the bite valve will not leak, even if you inadvertently sit on the Softflask or cram it into a gear bag. Hydrapak also offer the Softflask with a more sensible flip-top opening that isn't as quick to use but allows for much greater flow.
Overall, the Softflask may prove handy for casual long-distance riders who want to reduce the number of things they carry on a ride or racers who don't want to deal with tearing open a bunch of single-serve packets on the go. But in either case, it's still not as easy to live with as we'd like.