As the other poster said, they have advantages and disadvantages. Subarus and Posches use them (though in the Porsches, they're flat-6 engines, not flat-4).
The big advantage is center of gravity: their shape lets them sit basically flat, near the ground, and since engines are the heaviest part of a car, this gives the car a lower center of gravity, which is good for handling.
The main disadvantage is mechanical complexity: instead of all the cylinders lined up in a row, with the crankshaft on one side and the valves and dual cams on the other, you have two banks of pistons, each with their own camshafts. So now instead of two camshafts, two camshaft gears, and one timing belt/chain, you need double all those. This of course increases cost too.
Flat-fours also tend to have rather distinctive exhaust notes, which not everyone finds pleasing.
Another notable place flat-4 engines are used is in small aircraft. Lycoming and Continental engines for small (e.g. Cessna) airplanes and helicopters are all flat-4.
Porsche now makes a flat 4 engine again- the new Boxster/Cayman engines are turbo 4.
On the cost issue, Porsche famously tried to cut costs for the 996/Boxster gen 1 by using a common cylinder head casting for both sides. This was done by flipping it over for the other side. The only problem with this was providing the drive for the camshaft from the rear of the engine. To do this, they created an intermediate shaft operating at the rear of the engine. This worked ok but it couldn't have an oil fed bearing, so they used a roller bearing. This caused a failure rate if something like 5% of engines, and caused a class action suit that Porsche had to settle.
The other advantage of boxer engines is their suitability for air cooling and packaging. In fact, Porsche produced an aircraft version of the flat 6 but it ended up being too exprensive to build and run, despite outperforming a lycoming by a long way.
The big advantage is center of gravity: their shape lets them sit basically flat, near the ground, and since engines are the heaviest part of a car, this gives the car a lower center of gravity, which is good for handling.
The main disadvantage is mechanical complexity: instead of all the cylinders lined up in a row, with the crankshaft on one side and the valves and dual cams on the other, you have two banks of pistons, each with their own camshafts. So now instead of two camshafts, two camshaft gears, and one timing belt/chain, you need double all those. This of course increases cost too.
Flat-fours also tend to have rather distinctive exhaust notes, which not everyone finds pleasing.
Another notable place flat-4 engines are used is in small aircraft. Lycoming and Continental engines for small (e.g. Cessna) airplanes and helicopters are all flat-4.