Wow - lots of debate over the 5.7. I have a 2015 R/T Shaker with the 5.7. Its the A8 and obviously has STP, and is also a daily driver. My "daily drive" is about 20 miles of open highway and twisty back roads. No traffic, and just a couple of stop lights and only one stop sign along those 20 miles.
My previous daily driver was a 2010 Challenger SRT with the 6.1 - it had some mods (exhaust, CAI, strut tower brace, and tune). I bought the SRT new and drove it nearly every day for the 5 years I owned it, including one trip from VA to AZ and back. I've owned 3 Mustang GTs - to include a 2011 which we still have; 2 previous gen Camaros (Z28 and SS models), and as stated above, I'm on my 2nd Challenger. My first car was a 1968 Chevy Impala coupe (with a 350 V8, aka "5.7"); my second was a 1977 Chevy Caprice Classic 2-door coupe (variation of that year's Impala) with a 350 (5.7) V8. I've also owned a fun driving Jaguar with a manual transmission and an early, very basic Miata. Point here is I've got some time in a variety of V8 cars with a peformance orientation that span a period of 47 years - as well as some comparison to a couple of other popular "sporty" cars in either 4 or 6 cylinder.
So what do I think of the 5.7 Challenger in its current form (2015)? I love it. I like how it drives more than my SRT, which is why I bought it - and I really liked the SRT. Many of my cars had Chevy small block V8s (350/5.7), and while the horsepower and torque of each varied based on when it was made, etc. - they all ran nice. Good power and torque for the platform it was in at the time. I never felt underpowered as a daily driver - certainly not on the open road, where I've spent most of my driving life. Likewise with the Mustangs - they all had either a 4.6 or 5.0 engine - again, small block V8s - and were all pretty quick and torquey for daily driving and open road crusing. During my driving lifetime, there have been many cars that were faster, better handling, more powerful engines, etc. There always is and always will be - everyone knows that. For me, its all about how the car drives - my impression, since I spend a lot of time in my car on the open road. That's the reason I drive performance oriented cars...I like driving and my environment lends itself well to those who enjoy such cars. Best part is I'm fortunate that I can afford pretty much any mainstream performance car I choose to buy; American, European or even Asian. Recently, I chose the 2015 R/T Shaker with the 5.7 because it drives (IMO) really well.
Differences from my 2010 SRT? The 2015 feels lighter, as nimble and more refined in its overall performance compared to the earlier model SRT. Is it as fast? Not quite, but it has plenty of torque when I hammer the throttle and it handles very confidently when banging through the twisty back roads I drive daily (I did bolt in a strut tower brace the first week I owned it as I find all cars of this type benefit from them - regardless of manufacturer). Acceleration is excellent. In other words - the 5.7 mated with the A8 I find delivers solid performance in the 2015 R/T.
What attracted me to the 2015 model to begin with? I really liked the updated interior, was intrigued by the writings on the A8 in the Challenger line, and really loved the look of the shaker hood - so classic "old school" looking. I was not dissatisfied with my SRT - best driving car I ever owned up to now - but I liked the new R/T with the 5.7 better...the way it drives. Just today, I was smiling nearly all the way home wringing out the R/T on the back roads, enjoying just how well that 5.7 does perform as a daily driver on the open road. Could Dodge tune it up a bit and give us a little more HP and torque? Sure they could - but as a driver's car, the 5.7 R/T with STP does quite well from a performance perspective on the road IMO. I assume Dodge knows that - hence the reason they're still offering the 5.7 in the Challenger line after all this time. Just my impressions of the current 5.7 R/T - others will likely have other opinions. And that's OK.