Yep, I’d say they can be very worth doing, but only if the layout works as well as the loft itself.
The big things I’d check are stairs, not just head height. Plenty of lofts have enough height in the roof, but the awkward bit is where the staircase goes and what you lose on the floor below to make it work properly.
If you want a bedroom and bathroom up there, also check where the soil pipe and services would run. That can make a noticeable difference to cost and how disruptive the job feels.
Cost-wise, a simple rooflight conversion is very different from a dormer with bathroom. Broadly, people are often into tens of thousands rather than “small extension” money, and once you add a bathroom, steels and stairs, quotes can move quickly.
Disruption is real, but a decent firm will usually keep most of the mess in the loft until the new stairs go in near the end. That is often much easier than a ground-floor extension that turns the whole house upside down for months.
I’d also think about whether the house will still feel balanced afterwards. An extra bedroom is great, but if it turns the house into loads of sleeping space with a small kitchen and one living room, it does not always add value in the way people expect.
Best thing you can do now is get 3 like-for-like quotes from loft firms, not just one-off ideas from different people. Ask each of them the same things: what type of conversion they’re pricing, whether the bathroom is included, whether planning or structural drawings are included, and how long they think the build will take.
These gusy have a decent explainer too:
https://www.loftcompare.co.uk/loft-conversion-types