How good are LED panel lights for still photo work?  In a word, not at all.
   I’ve tried two - one by Manfrotto (below), the other by Litepanels.  And I’ve been dissatisfied with both.  Neither can provide adequate light beyond the shortest range - unlike a speedlite, and they obviously are intended for on-camera use, as each has a shoe mount.  And neither can work with any but a very narrow choice of lenses.  Neither is suited for macro use, either.
   Absolutely forget using either with your 70-200 or at any serious range; they also have problems covering the entire area your lens will for wider-angle lenses.
   Both run on easy-to-find “AA” batteries - and have easy-to-use dimming controls.  But neither approaches the capability a speedlite and inexpensive diffuser offers - but costs far more.  In short, buy a speedlite and diffuser instead.

Manfrotto ML360 panel LED light.