As far as I know, negative energy is theoretically still sound; I may be quite mistaken though. What "energy conditions" would negative energy (or mass for that matter) be violating?
These conditions are widely believed to apply to all realistic matter configurations (to the extent that the less-restrictive ones get used as postulates in some theorems in relativity), though there are some exceptions due to quantum effects (which I believe are usually short-lived). Just for example, a cosmological constant or other dark energy source does not violate the null energy condition.
It would be really cool to see any of the energy conditions violated for an extended time in a lab! I'm pretty sure, though, that it has not yet been done.
Kind of like you can compress ammonia (a bit) and nothing terribly interesting happens, and you can cool ammonia gas (a bit) and nothing terribly interesting happens, but do both at the same time and you've invented liquid ammonia and refrigeration and all kinds of nifty stuff.
On the other hand energy/mass seems rather fundamental. There are no multiple "easily" negatable components you can play with mixing together. There's no "add a to b to get the mass" going on. So you can't get sneaky with combinations and initial conditions to magically make negative mass/energy. It's really a different kind of problem.
The whole topic is pretty interesting to me because the first metamaterial stuff was constructed out of printed circuit boards with weird etchings right around the ham radio microwave 6cm and 9cm bands, which is right up my alley (doing weird ham radio things involving PCBs and such around those frequencies) Unfortunately I have been unable to figure out any practical engineering demo (like, an antenna or a waveguide or some such) that would be ham radio applicable in a real world demonstration. I guess I could make a really weird lens and place it in front of a 6cm horn waveguide antenna, but I can make a much simpler microwave lens antenna using about 1940s tech. Lens antennas using plates of styrofoam and aluminum foil work pretty well at 10 GHz ham radio band for example, but I digress.
Metamaterials are kind of a solution in search of a problem right now, like lasers in 1960s or home computers in the 80s or whatever. Most likely, much like the examples, it would be pretty hard to predict today just how important they might be in the future. Or maybe we're seeing the next "magnetic bubble memory" or magnetic amplifier technology or fluidic analog computing or whatever. Hard to predict.