I bought a Rav4Prime last year and am more or less happy with it. In EV mode the engine does tend to come on more often than the Volt's. It's not excessive, something like once or twice a week in cold weather (I noticed that now that I'm not using the heater, it happens less), and the most annoying thing is that there doesn't seem to be any logic as to when/why the engine starts and there is never a message indicating why the engine is on. The Volt always tells you (ERDTT, EMM, FMM). The only time it doesn't is if your hood is open. Anyway, I'd say the R4P is close to being a genuine BEV until the battery is depleted - not as close as the Volt, but probably closer than some other PHEVs. I've actually never had the engine come on when punching the accelerator, so it's not as if the R4P's motors are too small.
The R4P range is almost as good as the Volt's, and it routinely outperforms (I often get more EV miles than the stated range). And it's a fun drive, for a SUV. The Volt feels zippier, but the Volt is a small car. Of course the extra cabin space feels downright luxurious after the tiny volt. Another improvement over the Volt is that both heating and cooling of the cabin are performed with a heat pump, which is much more energy efficient than the Volt's resistive heating.
The only problem with the R4P at the moment is getting one. Dealers are marking them up by thousands.