The familiar phrase "Your Mileage May Vary" has never been better suited to any car than it is to the Fisker Karma. There are a number of long threads discussing this topic, none more entertaining than this one
The bottom line appears to be that if your charge-to-charge travel is short enough to be in pure EV mode, your gas MPG is essentially infinite (the Karma does fire up the ICE at regular intervals to exercise it and to check for emissions status, so it is not actually infinite). But if you have to engage the ICE, for additional performance or due to travel greater distances, the overall MPG tend to drop pretty quickly depending on the gas/battery ratio. The longest I have driven my car in a single day was around 260 miles, and by the end, the MPG was around 25, which is what you expect from the ICE alone.
Thanks AlexofSB... I work at night and it's all highway to the gig, so I set the cruise control at 65 for the straight stop with no stop and go traffic. In the mornings on the way home is the complete opposite though....
The critical questions for you will be: (1) how far away is each leg of the commute, and (2) can you charge your car at "the gig". If you can travel both legs on stored electrons, you may only have to put gas in your car once a year. In contrast, if you have to travel 100 miles between charges, then your MPG will not be stellar. It all depends on the route.
On the charging issue, if your car is going to be sitting idle while you are at work and you don't need to drive around during your work time to go to meetings or visit clients, etc., setting up level I charging from a regular plug would actually help a lot on your return leg. 8 hours of charging at 110V is good for almost half a battery worth of charge (25 miles indicated).
I was not really thinking about the effective MPG when I bought this car, but once I started driving it, the number of miles I could travel between fill-ups became an intellectual exercise and object of fascination and I found myself trying to drive as smoothly as possible to get the best mileage. Judging by the posts here, and the folks who sent in photos of their mileage to Fisker's Face Book page, I may not have been the only one.
Edited to add:
Here is another thread
where owners discuss their average MPG.
And just for the record, here is a screen shot from my energy flow screen showing my personal best mileage on one tank of fuel. Surprisingly, compared to some folks here, this is barely average.