They vary. My woman had a good one, but the company went out of business so there were no updates or support. She now has one from a well known company and it seems unable to cope well with the signal it receives.
(I know the signal she has is only just ok, so it should cope, but the signal should be better. However aerial experts she's paid to come look at her situation apparently can only fiddle in the loft for a bit, charge her for whatver they've done, and leave it with no real improvement.)