For me it was always Vanilla Android, until I got my s4. OG Droid, Dinc, and Nexus had vanilla Android through stock ROM or a custom ROM. Of course features in the custom ROM were baked in and made things a bit easier, but overall I do enjoy the clean look that vanilla Android has. My thoughts about it today....
Manufacturer UIs have greatly improved. Before I used to hate manufacturer UIs since they always seemed so "bloated" and the standard widgets used to be so crowded. TouchWiz, imo, used to make their devices seem like a Fisher-Price toy. Today, doesn't look like it. Sure TouchWiz still is colorful, but it doesn't seem as "childish" as it did before. It looks much cleaner than it used to and the same goes for HTC and Moto.
As for features of manufacturer UIs, honestly they're great. Makes our phones easier to operate and possibly skipping an extra step over vanilla android, it makes the experience a bit more fluid, especially for the people who aren't into rooting and running certain customizations. Camera features are far better than vanilla Android, and that's what many of the general public care about, camera quality/functions. Other features such as Air View, are features I thought were just gimmicks, but really do make the experience a bit better.
As for myself, I see no real desire to go back to vanilla android. I think the TouchWiz experience right now is solid, enjoyable, and makes things a bit easier. Yeah there's been a few hiccups, but nothing I wouldn't run into if it were vanilla Android either. Manufacturer UIs have come a long way as Android has evolved and instead of bogging down the speed of our phones they run smooth and better the experience. Maybe later on I'll mess around with vanilla Android again, but for now (and I can't believe I'm saying this as a former flash-o-holic), ROMs are not for me right now.