My husband and I are thinking about buying a 08 Mazda5 but noticed that they have an aluminum engine. How reliable and durable are they? We generally drive a car until it has 200-300,000 miles and that is our main concern.
They are fine and many modern vehicles do.
If you run a vehicle that long your driving/maintenance/awareness habits are good and anything else will last that long under your ownership.
You do not want to overheat an aluminum engine due to the risk of warpage. They also have a high head gasket failure rate compared to cast iron engines, although usually you will get 150,000 miles or more before they fail.
I prefer a cast iron engine , but it is difficault to find one these days.
GM made the mistake, many years ago, of manufacturing aluminum engines without cylinder liners of steel or iron, and the result was engines that burned oil at a very low odometer mileage. Nowadays, you don’t have to worry about that problem.
Of course, aluminum engines are more prone to problems if they overheat, but in terms of durability/reliability, the composition of the engine block has little to do with that factor nowadays.
I like cast iron… but I think there should be a line drawn here - there are different configurations you may see, most commonly:
iron block / iron heads
iron block / aluminum heads
aluminum block / aluminum heads
Personally, while I like iron, I would prefer aluminum/aluminum over an iron/aluminum combo. Aluminum and iron have considerably different coefficients of thermal expansion. Put simply, for every inch iron wants to expand when heated, aluminum wants to expand two inches. Obviously the heads and block must move together, so the difference causes a stress buildup that would not be there if the material used in the head and the block are the same.
That said, an engine well designed will not have significant problems with that mix.
But there is a reason why the Ford 3.0L Vulcan used on the old Taurus with its iron/iron combo is known to be VERY durable, while the 3.8L Ford Essex with iron/aluminum used on old Windstars and Tauruses (1995 and older) was known for headgasket failure…
The idea that the 3.8 failed due to differing expansion rates of the head and block is a myth, plain and simple. Ford put the head gasket fire ring very close to some coolant passages. Eventually, natural corrosion from the coolant passage side, and heat stress from the combustion chamber side, cause the gasket to fail.
It took Ford (and aftermarket companies) quite a while to develop gaskets that held up to the design of the engine, but they have (and Ford made a design change to the coolant passages later on.) Add to that the fact that many mechanics do/did not understand that the head bolts on these engines are NOT reusable, and you get the mess that resulted.
An aluminum engine is durable and will last a long time. The only caveats would be that the cooling system should be maintained properly and if the engine is overheating it is critical that the car should be stopped immediately, but that’s true of any car.
An all iron engine will tolerate some excess heat better than an aluminum one but if overheating is not in the equation one will last as long as the other.
The 4.6 DOHC in my Lincoln Mark VIII is all aluminum, has about 220k on it now, and still runs like new with zero problems. Oil consumption between roughly 3500 mile changes is barely noticeable; maybe a couple of tablespoons. It’s hard to tell.
Ah! The infamous Vega! They cast the blocks in a foundry in Messina, NY, I believe right beside an Alcoa plant which piped the hot aluminum right over. The problem was the coating they put on the cylinders to harden them; it just did not last, causing early oil consumption.
In addition, the engine block itself was used to radiate some of the heat since aluminum is a good conductor of heat. This allowed them to equip the Vega with a miniscule radiator.
Unfortunately, these engines both leaked and used oil, and dirt on the engine block soon destroyed its heat radiating capacity, causing more overheating, etc.
The rest of the car was subpar as well; GM’s halfhearted effort to meet the foreign competition. This car was a dog, and even the most dedicated maintenance could not keep it on the road.
One of our regulars owned a Vega, I believe. He will no doubt have additional war stories to tell.
The famous Toyota 22R and 22RE had an iron block and aluminum heads. My truck ('90) lasted to 265K miles before I had head gasket problems. The truck was 10 years old at the time. The second head gasket (Fel-pro) lasted until the truck was wrecked a couple of years ago, with 325K miles on it. Parted out what I could, complete with a running engine.
I had some worries, but the Ford Explorer we have has an aluminum engine with a plastic upper intake manifold. It has 160K miles on it right now, and still drives like a dream. Of course I was quick to tell my wife that if the engine temp EVER begins to climb, pull it over and park it immediately. We have AAA and cell phones, and a tow is far cheaper than a new engine.
I had a Cadillac with an aluminum engine. The General was a lot more problem than the engine. The Motors part seemed to run alright.
True, there was more to the 3.8L than just the aluminum/iron mixture. But anything which stresses the gasket more is certainly not helpful. That’s why I said a well designed engine will not have significant problems with the mix. The Toyota 22R and 22RE BustedKnuckles refers to was well designed. The 3.8L Essex was not. There was no excuse for that engine. 
But you’re right - you CAN make them reliable. But all else being equal, it is still certainly best to have identical materials just to remove that stress from the gasket.