I have a variety of fuels and mix them up during longer races. FWIW, I consider all of these as fuel, not hydration. For hydration, I'll consume the water or electrolytes being handed out at the hydration stations.
My feedback:
Gu gels: I like them; some tasty flavors and they don't seem to bother my stomach.
Glukose: Liquid and I haven't had any issues drinking them on the run. Tear the top off, put the opening to my lips like a straw, and drink until the pouch is empty. It's like a shot of fruit punch in a foil pouch. Looking at the Gu website, their liquid energy pouches look just like the Glukose pouches.
Maurtens: Unlike the two items above, the Maurtens does not require additional water in order to be metabolized. No real flavor to them and not the greatest texture, but gotta love the no-water-needed aspect.
Clif-bloks: I like them but consuming enough to make a difference is like trying to ingest the gummi bear from heck, plus water is needed. I'll take them when offered on the course, but I'll likely just tuck one or two of the sub-cubes (where they have the little separation lines) in my cheek and let it dissolve over the next mile or two. Ditto for a pack of the energy beans: I let them dissolve over the course of miles.
Based on advice from others on the forum, I've taken all of my fuel types out on longer training runs to make sure no surprises when I try to consume them during a race.
YMMV