I'm on the other side of the fence when it comes to all-season tires.
I have extensive experience between high-performance tires, all-season and dedicated snow tires on the same vehicle.
Vehicle 1: 2012 BMW X3 (wife's car)
We started out on the Bridgestone all-season tires that came on it stock. They were okay in terms of grip, noise and all around driveability in all weather conditions. Water evac was okay, not the best.
Then, I got her a set of Advan T5 wheels in 19x8.5 and put Conti DW performance high performance summer tires in 245/45. The road feel was much better and the propensity to hydroplane was greatly reduced. Overall, the feeling was much more sure footed. Oh, and the wheel/tire combination is 17lbs. per corner lighter with this setup. Yes, the stock X3 and non-runflat tires weigh a ton.
When the stock tires gave up, I put Michelin X-Ice Xi3 snow tires on. This tire is unbelievable in all types of conditions. I feel it's more of a hybrid rather than a true snow tire as in "normal" driving conditions where there isn't any snow and ice, the tires do fine. They are a bit soft, but perfectly acceptable. When the snow falls and it's really cold is where they are fantastic compared to the all-season. They cut through the muck so much quicker and easier to give you a better road feel and more confidence in the drive.
The Conti DW's wore out, so I wanted to get something that could overlap the winter months a bit easier than a true summer, high performance tire, so I listened to all those people saying that all-seasons are REALLY good now and you don't need a high performance tire. DON'T BELIEVE IT! I now have the Michelin Pilot A/S 3+ on her Advan T5 wheels. Compared to the Conti DW, they suck. There is no where near the feeling in the steering wheel as the DW's had. The grip is not as good (the sidewall is softer for sure) and the resistance to hydroplaning is not as good either. Overall, it's a definite step down. I wish I would have gotten either the Conti SportContact or another high performance summer tire to get the most out of the larger wheel/tire combination. I personally driver her car quite a bit and there is something lacking from the driving experience.
Vehicle 2: 2015 GTI (my car)
Stock it came with the Pirelli all-season tires. Okay, I guess when my car was stock, but once it was tuned, terrible. No traction whatsoever in the dry and the rain/snow was worse.
I got my aftermarket wheels (first Volk RE30 Clubsports then Volk TE37SL) and went to a 245/40R17 in Michelin PSS. Yes, this was a major difference! Wet or dry, they are night and day better than any all-season I've had on the car (and I'm comparing to the latest Michelin Pilot Sport A/S 3+ that I have in stock size now for the winter.
Winter, I put all-season tires on as I wanted the option to run these longer than a dedicated snow tire like my wife's X3 has. Again, I went by what everyone else was saying to run the all-season tires. What a mistake! The grip in the slush and snow is horrible. Steering response is horrible and doesn't cut through the snow the way it needs to. I'm very disappointed! I will be getting a dedicated snow tire the next time. In the dry/warmer weather, the all-seasons are fine. They are an improvement over the stock Pirelli's for sure, but they are not even close to the Michelin PSS I have for the summer. I can't imagine the difference with the new PS4S that are probably going on my car next.
Vehicle 3: 2006 BMW 330i (previous car)
The stock Bridgestone RFTs where absolute crap!
I changed to wider Michelin PSS over the last few years that I had the car and they were absolutely stellar on the street and track.
Winter time, due to it being RWD, I switched to Bridgestone Blizzaks and it was like my car had AWD in the muck. One dry pavement when it warmed up, they were soft as soup noodles and didn't handle worth a crap, but that wasn't their intention.
In the end, I have first hand experience with all-season vs. summer vs. winter tires and I can tell you, there is a pretty significant difference. If you want a high-performance car to work at it's best, get the best you can get. They don't put all-season tires on Porsche's, Ferrari's, Lamborghini's and Corvette's for a reason.