I've tried a variety of "ultralight" (e.g. bottomless) sleeping bags and other bags that aren't "ultralight" per se but are specifically designed for backpacking, and thus emphasis weight and space savings. I've had bags from Big Agnes, Mountain Hardware, REI, North Face, and Marmot. I currently own 4 "backpacking" sleeping bags.
I've determined that one inevitably has to make compromises, and its really a matter of personal preference as to what the ideal set of comprises is for one's specific situation.
For me, I sleep cold and I'm a side sleeper that switches sides every couple hours while I'm sleeping. However, I'm not a big guy, so I can't carry weight like its nothing, but an extra pound in a sleep system so that I actually sleep every night is totally worth it. (I'll bring a pound less food and forage if I have to!). I'm also value conscious (not cheap) I look for high QPR for the dollars I spend, and if that means the most expensive, then thats fine with me, but often it means the lowest total cost when all factors are considered. Versatility and multi-use is always a big advantage for me.
Because I'm a side sleeper, I cannot sleep on foam pads now that I'm in my 50's. At 20 I could sleep on a rock and it didn't matter; now I need some padding. Thus an insulated inflatable is my go-to. Currently I use an insulated Klymit Static V Lite and it is very comfortable and I have not had a problem in two seasons of use with leaks. I always bring repair materials. What I like about it is (1) the baffles actually seem to make it more comfortable (2) I slide around less on it than I do on something like a NeoAir, AND (3) it is very quiet in comparison (no potato chip bag noises in the middle of the night).
For a bag, I always buy 0 degree bags for versatility and because I know anything rated at 20º will actually have me freezing when it's 30º outside. I recently discovered a North Face bag called "The One Bag". It's about $250-$300, but it is the most comfortable and warm bag for its weight and size I have ever owned in 40 years of backpacking. I own three other bags (down and synthetic) and I always choose this one now when I head out, unless its a car camping trip with my wife in which case I bring our Back Country Duo bag.
What I love about the One Bag it is that it is convertible - 3 bags in one, really. For summer, you can use it as an ultralight, synthetic fill bag rated to 40º. For Spring and Fall, you swap the synthetic top for a 800 fill power down top and the rating goes to 20º. For winter or cold conditions, you add the synthetic top back on to bring the rating to 5º. I've slept in it down to 18º with just lightweight merino base layer and fleece top and balaclava and was comfortable. The bag weighs between ~ 2 lbs and 3-1/2 lbs depending on the configuration and the bag is wide enough to allow me to sleep on my side with my knees bent and to switch from side to side without feeling like I'm fighting against anything (unlike more constrictive bags I've had).