I also had to Google! Pic attached for the benefit of other users...
I have a 2.5 year old and I'm currently expecting another baby. Now I know that these things exist, if I had the space, I'd buy one! Babies who are starting to climb etc would need close supervision, but they'd need that regardless...
It looks versatile and fun, would help teach climbing, pulling up, balance, coordination, etc. The slide addition would be great for most ages. Older children could convert into a den with sheets. For any future younger babies it could be used as a baby gym (e.g. clip pram toys to the bars and put them on a mat underneath).
For what it's worth, I'd choose a pikler triangle over a plastic slide (far more versatile and more uses = better value), plus you could add a slide in future in any case. Foam crawling shapes are fun (my 2.5 year old enjoyed them at soft play when he was a baby) but of limited value as children start getting more agile (they would have been packed away in the loft by now, whereas the triangle would still be out, if I'd bought them) so again, the triangle would offer interesting options for longer.
Useful? Yes.
Dangerous? Most things in the household have the potential to be dangerous. It's about risk assessment and supervision. Just make sure you buy from a reputable brand/seller who spaces the bars safely (you don't want little heads getting stuck).
Too expensive? I think they seem like good value, given the versatility.
As an aside, does your DH prevent you spending money on things, or is it this just a topic of conversation because it's a large item (both in terms of floor space and money)?