I started writing a reply to actually address the topic of Estates, it's kind of turned into a mini strategy article. Oh, well.
TL;DR Combos with Scout.
Of course Estate is not a great card. That's why you start the game with three of them in addition to seven Copper, and it should be your aim to get rid of them as soon as possible. Despite that, there is a certain level of strategy relating to getting Estates into your deck. The two big things to think about are (1) why do you want Estates, and (2) how do you want to get them?
(1) Why do you want Estates?
In most games, the answer is either points or piles (or both). Occasionally, there's a functional use - Baron or Inheritance, for example. But the fact that Estates are a cheap card worth Victory points means you don't discount them in other games. If you have lots of +Buy and a decent source of income (or can get a decent amount of cost reduction happening), then a three-pile ending is presumably in the cards, and Estate is an obvious option for one of those piles. Even moreso if there's an alt-VP source that benefits from having those Estates, the clear frontrunner being Silk Road.
As a source of points, while definitely not the most efficient, it can still sometimes be one to watch. Many people have stories where they won or lost a game because someone noticed they could three-pile and grab a single Estate. Even a Big Money player might, in the last few turns, want to dip into the Estate pile to get themselves a winning margin.
(2) How do you want to get Estates?
Again, barring something like Inheritance, the answer is presumably "in bulk, right before the game ends". Because whether you want them for piles or points, you're happiest when you can get as many of them as possible in a way that doesn't clog your deck. So then, the question comes down to whether you want to buy them or gain them another way.
The gaining options are Workshop, Ironworks, Smugglers, Horn of Plenty, Hermit, Armory, Dame Natalie, Altar, Artificer, Feast, Duplicate, Baron, Followers, Border Village, Hunting Grounds, Messenger, Alms and Ball. The gaining options that are worth considering are Baron, Hunting Grounds, and maybe Artificer. In a pinch, you could also look at Workshop and Ironworks, and Inheriting Ironworks is ridiculous but also kind of amusing. The one thing these mostly have in common is that you've probably used them for something else first, and it's just because you're on the last turn of the game and want to creep across the line in points that you might use them to gain Estates. Artificer is probably the most spammable of the lot, unless you've got a good way to trash two or three Hunting Grounds, but in the presence of ample +Actions and/or King's Court practically all of them are usable.
Buying, on the other hand, is just a matter of accumulating sufficient +Buys and coins or coin equivalents. If you're building an engine, you've probably got both of them in spades anyway, and it would be rather pointless to make a complete list. Some notable examples, though - Grand Market, each play is another Estate in your pocket; Bridge and either a non-terminal +Buy or a suitable Village, especially if you realise your megaturn isn't going to come fast enough; Candlestick Maker - has the double advantage of providing another cheap pile to empty; Worker's Village plus a modest source of +Coins.
There is also a third option - why not both? Especially if you've been using something like Ironworks to gain engine pieces (like, say, Worker's Villages), then if you can gain half the remaining Estates, then buy the rest for the three-pile ending, then check your point count because that may just win you the game.
All of the above mainly applies in an engine situation, of course. In Big Money, you typically only dip into Estates when you're really sure you're close to the end and you won't see them again, and in a slog you get them whenever the opportunity presents itself. Combo decks, well, it depends on the board.
(3) The other cases
The other cases being Inheritance and Baron, mostly. And to be honest, Baron is not at its strongest in a deck full of Estates. Baron is at its strongest in a deck with one or two Estates that you can keep drawing and discarding. So the less you trigger its gain ability, usually the better, although it can be great in a slog. Inheritance, well, you probably want to have already Inherited or be ready to do so before you start gaining Estates en masse unless you need to win the split and you think your opponent is about to grab a bunch. Inheritance is just fun times all around, and it deserve a completely separate strategy discussion (which it has). I haven't even played with Inheritance so I don't want to theorise too much in the presence of people who actually know what they're doing.