I keep looking, as I have a tiny amount of grass to cut. But don't think they are close to being viable yet, looking at reviews - most people complain that if the grass is thick or more than 2mm long, it struggles, and that kills the battery.
Also, battery care is a problem. They all use proprietary expensive batteries that last about a year, and then its almost as cheap to buy a new machine. Obviously, you cant store lithium batteries in a garage and expect them to work again after a cold winter, so maybe thats part of the problem?
I half eyed up the Ryobi one advertised on telly, as I assume it uses the standard Ryobi batteries, and comes with a strimmer as well for about £200... ...and my current Wank and Decker cordless strimmer is shite (the old NiCad based one was excellent, and lasted about 8yrs (with regular battery replacements)), even in "Turbo mode", which should be classed normal mode, as normal is a waste of time, and just a stunt to allow them to claim a 20 minute run time.
But I'm sticking with my B&S engined Hawk for now, as there is nothing better available. Remember a stupid mains powered mower can't match the crappiest petrol one, and it appears a battery mower can't get near even a mains powered one.
Which is how far backwards we seem to have come - Dad had a battery powered cylinder mower in teh 70s, and on a single charge of its (car) battery, it would easily do both reasonably sized lawns.