We have https://www.happybeds.co.uk/tyler-grey-wooden-day-bed-with-guest-bed-trundle-frame-only-3ft-single in white.
I can't tell you how good it is as we haven't had cause to use the trundle yet, but we just bought the bed and used the two mattresses from the bunk beds we had before. The upper bunk mattress was a little thinner (as per regulations) so that was the one we put in the trundle. They are IKEA mattresses and really comfortable.
Obviously there are downsides as PPs mention; there's no headboard to the trundle so you may lose your pillow overnight, and it's awkward for the person in the main bed to get out to the loo if they need to. (Best put the person with the weakest bladder in the trundle...)
I'm still happy with it as a compromise for providing another bed, I considered a sofabed also but we needed a bed for DS primarily then additional sleeping, so this works better for us. In a spare room a sofabed might be better.