I have an 06 M35 Sport and love it. It handles like a true sports car. At 100+ mph it feel like it is on rails. The ride is a little stiffer of course but that is what the Sport edition is for. Fuel economy average is around 21 which is fine for this car. Seating is comfortable and roomy. It has 120K miles and still drives like it has 10K. If you are looking for a 4 door sedan that drives and handles like a sports car not sure you could go wrong with any of the Infiniti line up.
Car now has 168K miles and still runs great. Only issues I have had is headliner has dropped in the rear of the car, replaced water pump (regular maintenance), and the engine uses a little oil. The oil issue is common with those engine. Still enjoy driving the car.
Bought my pre-owned '06 M35x last Spring ('11) with roughly 60K miles - Had done a lot of research and had owned an older I-30 with 275K miles (replaced tranny @ 230K miles) - Had great luck, wanted to keep the brand.
During my research, I kept hearing others mentioning the Infiniti Elite Extended Warranty - I had heard about a dealer in Arizona (Scottsdale I believe) that had the best prices - I dealt with a lady named Courtney (Finance Mgr.).
I didn't/don't have a ton of discretionary $ - So the $1600-$1700 for the extended warranty was a hard pill to swallow - Especially for my wife - But we "found" the $.
Have MORE than recouped the $ invested (both frt. wheel bearings, CD plyr)
Likes: Looks (outside and in), technology (laser guided cruise, lane departure warning system, BOSE 16 speaker system), Nav system (split screen feature looks sharp), acceleration, heated and COOLED seats (hot summers in Georgia!), service from dealer (with ext. warranty - I know they are not cheap, but I wouldn't think of buying a car this technological without a reserve plan).
Quick story: A magical pebble leaped up/was tossed up @ highway speed and hit my radiator @ the perfect angle to pierce my radiator and the A/C system tube behind it - This WASN'T covered by warranty - Dealer SPLIT the cost of repair with me to earn my biz/trust. Didn't have to - In my book, that's good cust. serv
Dislikes: Better MPG, improve build quiality some (should both front wheel bearings / hub assemblies be going out @ 70K miles?), drivers seat heater is SLOW to warm up (gets a lil' cold in GA, too).
Overall I am very satisfied with my 2006 Infiniti M35x I brought it recently in August 2011 with 86,000 Miles on it and I currently have 98,000 Miles (Highway Miles) So far only had Minor issues. The first problem I had was the drivers side wheel bearing went bad $400.00. I had a very small oil leak but it was from Jiffy Lube forgetting to put back on the gasket for the drain plug. And my driver seat is broke but operational to drive the car safely needs a seat bracket and a new motor $1100. As far as the power train very nice acceleration but not as strong as the M45. Engine is loud but I like the way it sounds like a muscle car. Smooth ride overall very reliable on long road trips.
Love the Navigation and bluetooth feature rear back up camera all the buttons and gadgets. Car is well loaded even for the standard version, I dont have the adaptive cruise control feature love the headlights they move with the way you steer. The 2006 and 2007 models dont have the iPod AUX feature installed only the 2008 and above so I installed it myself the unit is $200 on ebay or at best buy but dont get the Gromin one on ebay it sucks search ebay for a good one the Nissan brand one works great. I have the isimple unit from best buy works alright.
The main improvements I would like to see on this car would be a 6th gear on the trans because the RPM is too high (3000+) on the highway at 70MPH or so. Also better gas mileage which a higher gear will help especially for the M45 owners. Other than that Great car I plan to keep it for 4 to 5 years or 275,000 Miles which ever comes first.