If you have never driven a luxury car and are used to the small, economy cars you will have to make the necessary changes in driving style. Not all luxury cars are huge, some are not much larger than a standard car, in which case the adjustment won't be too great.
Luxury cars, like all car rentals, are typically less costly to rent per week or longer rather than per day. It may be important to consider if renting for an extra day to make up a full week may actually be cheaper in the long run.