yes. They can be useful, particularly for births, deaths and marriages, but they can also be kind of random. In the 19th century lots of village stuff gets into the papers; in the 20th, very little does. My peasant ancestors in Britain are seldom mentioned, but a bookseller one frequently does.
Plus the actual selection of papers is variable, though it's growing all the time. Check and see how many papers are actually included in the times and places you want.
The other problem is that their optical readers are very unreliable, especially but not only with older papers where the ink has run a bit. So your grandmama could be mentioned lots of times but if her name has been mistranscribed you won't come across any of them. So try to search in as many different ways as you can - first name, surname, name of town, name of street, anything you know for sure.
And sign up for the emails. Two or three times a year they offer a month's subscription for £1. (If you sign up for that, make sure you go into My Account and untick the auto-renew option.)