This usefulness lasted up until I decided to search for nearby places. Now, every location-based site since, oh, the turn of the century has both list and map views for such results, because maps are an incredibly useful way to show an overview of the results but lists are better for details. Except the National Trust site, which contains no maps whatsoever and displays a grid of tiles that may or may not be sorted in any useful fashion (certainly there's no way to control the sort order). So it's impossible to see which places are actually nearby and which aren't at all (like anything on the Isle of Wight, because that involves crossing the Solent). It's also very hard once you've found a place to work out where it is - the directions page has a link to Google Maps, which drops a pushpin with an unhelpful name (like "1 Church Cottages" for "Hinton Ampner").
And finally because that's not enough fail, searching for events lists the results sorted by location. Not date. And there's no filtering, so I actually can't search for events during a specific time range - instead I have to scroll through pages of irrelevant events that don't start until the middle of next year. Sigh.