I've been using /e/OS on a FP6 for a few months now and I'm very happy with it (edit: for context I'm in Europe). I had doubts about whether it could run certain apps like bank or government apps but they work just fine. Most of the default apps work well, but I did replace the launcher because the one that comes with /e/OS doesn't have as many features as I would like.
Battery life is also better than previous phones I've used. I don't know if that's because of the hardware or because the OS is less resource hungry.
One thing to watch out for is it doesn't have a headphone jack, though you can get a separate adapter that plugs into the charging port.
It may be because the basic account is so much cheaper (€1/mo) that they offset it with higher prices for additional storage. Just out of genuine curiosity, why would anyone need that much storage for e-mail?