There are several distinctions between the GM/Chevy 3.5L V-6s, plus between earlier versions of that engine family.
First, the 3.5L V-6 debuted in the then-new Malibu a few years ago. "Vortec" technology applied to the cylinder heads aided both power and fuel economy. V-6 power with the same or better EPA ratings as some highly-regarded import 4 cylinders. I did a short 50 mile run in a rented Malibu, using the cruise on freeway travel, plus to vary the speed up and down with the cruise. At the end, the "Average Fuel Economy" readout on the trip computer was going between 36 and 38mpg. That was basically at a 60 mph cruise.
The first Chevy 3.5L V-6 was rated at about 200 horsepower in the Malibu. It was not until 2006 that it went into the Impala (replacing the Buick 3800 and prior Chevy 3.4L V-6 engines). The 2006 Impala version was rated at 211 horsepower and has "cam-in-block" VVT. It also has an EPA Highway mileage rating of 31mpg. I rented one with a few hundred miles on it and at the end of a 200 mile trip, it was reading a solid 30mpg on the trip computer (running with traffic on I-45 south of Dallas, which does not always run at posted speeds!) and taking no great pains to drive it for max economy. When I dropped it off at DFW, it was on a solid 31mpg. Many "Instant Fuel Economy" readings (on flat roads) were in the middle 30s mpg range. I was impressed! The VVT has expanded the middle and upper rpm range power, plus pumped-up the low and mid-range torque. Not a bad deal at all! Many potential buyers of Malibus have stepped up to Impalas when they see the fuel economy ratings and then drive the cars.
Not sure about the stated "hard start" issues. With the starter now controlled by the ECM rather than just the ignition key, some seem to have a little too much delay in them for what I'm used to. It's all computer-run now. A member of one of the car clubs I'm in bought a new Maxx a few months ago. He loves it. Fairfax Assembly even called to see how it was doing for him, which impressed him.
The other new version of the Chevy 60 degree V-6 family is the 3.9L V-6. Introduced in 2006 on the 3LT and LTZ Impala models. Initially rated at 240 horsepower (the same power as the prior Buick supercharged 3800 V-6, but without the greater torque of that supercharged Buick V-6), it was also VVT. For 2007, it also gained "Active Fuel Management"--which can be monitored from the Driver Information Center's display. A good running engine!
With the Active Fuel Management, the stated benefit is an additional 8% fuel economy gain. Only issue is that it's going to be variable for the terrain you drive in and whether or not you generally use the cruise control on the highway. You'll never know when it's in 3 cyl or 6 cyl mode, from the way it feels, but you can see it on the Instant Fuel Economy display (which includes the number of cylinders in use at that particular time).
From my experience with reasonably new rental Impalas and Malibus with the 3.5L V-6, I think it's an improvement over what they used to have. I also think that unless I needed something in the Malibu's size, I'd go ahead and get an Impala instead.
I should also mention that the Chevy 3.5L is NOT the same motor as the DOHC 3.5L that was in previor Oldsmobile Intrigues and Auroras. THAT engine was a Northstar derivative and is now out of production. As it progressed, it became more responsive and fuel efficient from the first ones, but it went away when Oldsmobile did (which was it's only use).
Just some thoughts and observations,
CBODY67