My company car is an Impala LT, and this is my third and I've liked them all. I run them to @ 65,000 mi. The two previous were 3.5's and they had good power off the line, but fell flat after that. But overall I found them adequate. I averaged 27mpg with them.
I've got @ 1,200 mi on my '12 w/ 3.6, and wanted to vent about it. In a roll-on situation it's a beast. Great power, smooth, and the engine is so quiet I hardly know its running. The six speed is great, with little gear hunting. But...that darned tall gear ratio kills it off the line. It's actually almost a dog off the line. I actually hate pullling out in front of people at times. The 3.5 was not like this at all. It had grunt off the line.
But what is really bad is in reverse. If you have to back up a hill, it almost WILL NOT GO. I had to pull into a fairly steep driveway the other day and - and I am not kidding - almost had to floor it to get it to back up. The engine revved and the torque converter just whined. Even on small inclines the tranny groans in reverse and you really have to gas it to go. To those that may not know better, you actually think something is wrong with the transmission.
Anyway, just passing it along. If the 3.6 had the 3.5's gearing, it would be awesome. The gearing just takes the balls out of it, and in some situations give some real funtional issues.