My Zipp 303S were a very noticeable step up over my Reynold's Assualts with the same tire, though the internal width difference was pretty significant (23mm hookless vs 17mm hooked, respectively). They're definitely more comfy (modern 28mm @ ~60psi for me at ~180lbs) and so much more surefooted when throwing them in to a corner. The tire/rim transition is much, much smoother; while that's supposedly more aero, it's also beyond the resolution of any seat-of-my-pants measurements. They were also much better bang-for-the-buck, though there have been more manufacturing advances/differences than just hookless vs not in the 5 years between them. To me, the transition from Assaults to 303S has been, for lack of a better way of saying it, a handful of marginal gains, within one product, that have made a significant overall difference. Along those lines, a little more than a year ago, I went from a Domane with Assaults to an R3 with 303S. Both were improvements to me, but if I could choose only one of those upgrades, I'd be hard-pressed to make a decision - I think that's saying a lot.

Anyway, my overall take is: if you've bought in to wide (28mm and up) and you've bought in to tubeless, seriously consider them. If you're against or on the fence for either, then sit it out and don't worry about it.