These are great, excellent choice for an elec assist bike. I had the completely uncool step-thru version with the not so good nexus 7 speed.
Most elec assist systems use a hub motor which is usually cheaper but is only really efficient around a particular 'average' speed. The crank motor on these means the power runs through the gears which is much better for hill climbing or load carrying. It gives a nice extra boost as you begin pedalling which is great for accelerating from a stop. Though you do need to be a bit careful at roundabouts.
With 18ah battery you should be fine for 50 miles. I had 12ah and got around 35 miles.
The legal limit for assist is 15mph. You can ride as fast as you like, but the assist stops at around 15mph. So above that it's all up to you.
It would be difficult / complicated to de-restrict electrics, but as the power runs through the gears you can increase the speed at which it will assist to by simply changing rear cog for a smaller one. Of course that raises the ratio of all the gears so you'd need to check that'd be OK for your normal riding area. If you like it and it's a keeper you could always upgrade to Alfine 11 and have both lower and higher gears.
Really great for commuting as on days you wouldn't be bothered or couldn't face the headwind etc you still get out there riding.
DO follow advice for initial battery conditioning as it will make a big difference to range.
The one thing I didn't get on with is that the power available is also related to cadence with max power around 40RPM tapering off to no power around 65RPM. In practise that means that sometimes when hill climbing you're better off changing up a gear to decrease cadence and increase the power. Pretty counter intuitive and I didn't like it.
It works fine for most of the target market, but most folk who cycle regularly prefer higher cadence levels.
I now know there's a fix for that which is to change the motor sprocket. This moves max power, and the power down phase, up the cadence range and would feel more 'normal'. I really wish I'd known about the fix before I let the bike go!