I'm 6 months into P550e ownership. For me it's a perfect combo. Almost all local miles are done in EV mode, making as much use as I can of the off peak rate here in the UK, albeit with a slow 3.8kW 16A charger.
Here's my example calculation... My "worst case" scenario is when I have to charge from empty in one night, which means I can charge ~19kWh in the 5 off-peak hours, and then pay full rate for another 15kWh (battery has only 32kWh usable capacity, but you lose a few kWh due to charging inefficiency), but even then that is under £6 for 55 miles of electric driving. GIven the cost of fuel, it's the equivalent of 60 mpg in ev mode. If I'm organised enough to charge only during off-peak hours, the ev mpg equivalent rockets to 200mpg. If I installed a proper 7kW home charger, rather than using the 16A socket I had for garage tools, I'd be able to charge 100% in one off-peak session, but it would take me a long time to recover the £1000 installation cost (around 250 mixed peak/off-peak charging cycles, so at least a couple of years).
Real fuel economy running on petrol only is around 30mpg (Imperial rather than US gallons), not bad for a 2.5 tonne SUV, and just as good as my old Porsche Cayenne S. In the last 6 months, we've done 2:1 ev vs. petrol mileage, so we're comfortably beating diesel fuel costs, and we still have the ease of fuelling for long trips with the UK's slightly patchy and expensive public charging infrastructure.
To cut a long story short, definitely do your own maths based on how you'll actually charge the car and your ev/petrol mileage mix, then weigh up subjective points like whether you would enjoy wafting around in silent EV mode, or feel it's worth having zero tailpipe emissions in town. I'm enjoying those side benefits even as a committed petrolhead, and certainly enjoying having 550bhp and instant torque on tap.