I bought my j30 from a one time owner who maintenanced every 2-3 weeks. (I know it sounds ridiculous but he had all the receipts). It's a wonderful car, and it's in pretty good condition. But as with all cars, time wears them out. The problem there is the cost of everything. (these are what I paid at the time, may be different) 80 dollars for a rebuilt waterpump, 600 for the labor. Also have had problems with the stereo, 1100 for the plugs, brakes, and struts. Overall car is good and has lasted well. If regularly maintained and taken care of, it's fairly decent. Just a few minor issues with electrical things, but long lasting vehicle for sure.
I've loved J30s since they came out. I bought my J from its original owner who took very good care of it. It has a beautiful interior with high quality materials. I've heard some people say these cars are like Maximas, but they are definitely of a higher caliber with respect to fit and finish, ride, and noise level. The engine and trans are very smooth with adequate power. The ride is smooth and comfortable. I have not had to do anything other than regular maintenance.
I own a J30t that has been my daily driver for 4years now. Very reliable, sturdy handling and strong powertrain. The car is esthetically very pleasing yet I can't help feeling like I'm driving Grandmas car. I meant to buy the Q45 4 years ago and got impatient for one to come along for $5000. I bought the J30t with 59000 k miles in perfect condition and so far its proved to be a very reliable car. No problems except for the factory stereo went out but I replaced with new stereo that's compatible with the factory Bose speakers. Dash light switch is querky, and I had to replace a relay in the Headlights because one night they just randomly started blinking off and on.
Bought J30 in 2000 w/80K miles. Mileage concerned me, but test drive hooked me. Felt solid, expensive, & fast. Now 14 years old with 170K miles on it. Mechanically, has not spent a day in the shop other than oil changes. Most reliable car I've ever owned. Had to know if 150mph speedo was fluff, so one time I took it up to 140 mph and it didn't miss a beat with the 300Z engine. Very impressive. Only flaws: rear end will break loose on you if you corner too aggressively, and isn't worth a hoot in snow (needs sandbags in the trunk in the winter). Sunroof failed so I sealed it shut. Auto door locks and dash lights malfunction occasionally. Exceptional build quality & no body rust. Keep forever.
I love my infiniti, I bought it with 115000 miles. When I got it it would not run in cold weather or rain, I replaced the Temp Idle sensor located next to the the thermostat, took care of it. Previous owner said he spent 1200.00 trying to fix the problem with dealer, never got it fixed, so he sold it for $1000.00. Great investment.