They may be, it depends on a lot of different factors. The market is becoming very saturated with copies of Quantum-style battery packs, which is reducing the prices, and they certainly do the job - but you still end up with a hotshoe flash with all its limitations, which may or may not matter to you - it depends very largely on the type of work they have to do.
For sports/press photography they can be ideal, but you'll probably need to cobble a few of them together to get enough power (and you'll certainly need to if you want enough power to overwhelm the sun) which will be very expensive as well as complicated, and if you have to use them at high power settings you'll lose the advantage of short flash durations.
But hotshoe flashes struggle to produce quality (as opposed to quantity) of light, and if you want to shape and control the light well you really need studio flash - which comes back to a battery and inverter but, as Jonathan said, user reviews seem to be a bit thin on the ground and it's difficult to find out just how well they work in the real world.
To me, the obvious solution is battery powered studio flash, such as the
Lencarta Safari Li-on or the
Elinchrom Quadra Ranger. Both of the ones I've linked to are twin head kits, you pays your money and you takes your choice.
