I owned a 2003 G35 for 4 years and currently own a 2007 G35x which I've owned for about 2 1/2 years. The 2003 was purchased with 108000 miles on it (173000 Kilometers) and I drove it till it reached 193000 miles (309000 Kilometers) During that time I spent about $2500 in repairs which I felt was very reasonable given the high mileage on the car. The repairs were for crank and cam sensors (whatever they are) and a new EVAP vent valve and cannister (emmisions equipment)
The 2007 has proven to be even more reliable, though I purchased is with fewer miles on it (59000 mile or 94000 Kilometers). It currently has 100000 Miles (160000 K) and I have spent a grand total of $125 on repairs! (leaking radiator hose).
The powertrains in both vehicles performed flawlessly and the cars are very fun to drive. Great power and handling and very comfortable. I actually thing I liked the 2003 a little better, but they are both great cars!
Hope this helps with your decision, best regards.