I bought the NeoAir because I wanted an inflatable mattress that kept the ground cold from getting though to my body, I've found it does this very effectively. I bought the fitted sheet because I tend to only wear boxer shorts in the warmer months and I'm not keen to sleep directly on NeoAir plastic, plus it cuts down noise when I turn over during the night. My motivation for buying my sleeping quilt was twofold, I wanted to lower carrying weight and to buy something that compressed into a smaller size. having no hood and no back on my quilt solved both problems. Obviously nothing is perfect, but I began to notice the cold a bit as I've become older and I think my quilt perhaps has lost some heat retaining qualities over the years that I have owned it. I thought of various ideas (New down, wearing more clothes overnight etc. I do not want to spend out money on another quilt) so in the interim I bought a simple cheap (£5.99) throwover camping blanket from Decathlon. I used it in the Lakes recently, worked a treat, I was so warm both nights, I had to throw it off me from about midnight until about 5am. I then felt chilly again and pulled it back over me! It never occurred to me that my outlay would be less than £6, I was prepared to pay more, now I don't have to.
I tried a friend's quilt, a Cumulus I think, and didn't get on with but then I suffer from a lot of back and neck problems so getting comfy is hard and consequently I move a round a LOT before I finally fall asleep and I found that that it kept creating little gaps and draughts and I couldn't just roll about without thinking about how it was "sealing" back up again. I was thinking, whilst using it, that it would have been better if it had a back that attached with velcro or something rather than all the elastic, converting it into a bag but with no insulation on the bottom - just a bit of material. I know plenty of bag makers do a differential fill but they don't seem to do a zero bottom fill for some reason.
I looked into quilts a fair bit before finally buying my last sleeping bag and it seemed to me you save a bit of weight - like c.100g on a -4C limit bag. But if you then need a liner, sheet or blanket then the combo is heavier, potentially quite a lot heavier. So In the end I spent a reasonable amount on a really warm winter bag and then got cheapo Chinese envelope-shaped bag for summer which can be used as a quilt when opened out and I can add a liner or blanket if need be. That way when it really matters (winter) I'm 100% sure I'm gonna be warm but I save a bit of weight in the summer and get a bit more flexibility.
I had a night a few years back where I used an old bag which wasn't up to the very cold night and was dangerously cold and I vowed never to let that happen again.