I tend to use them when I'm looking to buy something of category X, and want to get a market overview. I can then still dig in and decide whether I consider the sources used trustworthy or not, and before committing money, I'll read some reviews myself, too. Still, it's a speedup for me.

Yes, this is one of my primary use cases for deep research right now. It will become garbage in a few short years once OpenAI starts selling influence / ads. I think they’ve started a bit with doing this but the recommendations still seem mostly “correct”. My prior way of doing this was Googling with site:Reddit.com for real reviews and not SEO spam reviewers.

Same case for me. I find it pretty good at it too. Far from perfect but it usually a pretty darn good start.