Pros: 55-inch reach, safe to bury, can be connected, fits both standard downspout sizes
Cons: not really pretty
Water is the bane of the homeowner's existence: if you're not fighting to keep it in the plumbing where it belongs, you're waging a losing battle to keep it out of where it doesn't belong. Among the homeowner's arsenal of weapons for the second is the gutters and downspouts that capture rainwater from the roof and - you hope - direct it away from the foundation. Let water get too close to the foundation if you have a basement, and you'll have a moisture problem. If your house is on a slab, uneven moisture distribution around its perimeter can cause cracking and other damage up to partial collapse of an exterior wall. The flatter the landscape, the further away you need to dump the downspout's output.
If there's a modest slope away from your foundation, a splash block is usually sufficient, but if the terrain is flat you need more than a mere couple of feet. For several of our downspouts, we've installed an Amerimax Flex-a-Spout; a better alternative because its 55-inch reach is more than twice that of splash blocks.
Amerimax sells Flex-a-Spout in four colors (white, green, brown, and stone), but they are otherwise identical. The product is made from heavy-weight polystyrene, and sort of resembles a concertina on steroids. Most of the length is accordion-pleated, allowing the spout to expand from 25" collapsed to 55" extended, and also to curve sideways if needed. Rectangular couplings are molded at both ends, allowing it to fit either a 2"x3" or 3"x4" downspouts. For more secure installation, you can use self-tapping screws to fasten the spout to the end of the downspout. If 55" isn't enough, two or more Flex-a-Spouts can be snapped together. The spout can be buried, and holds its shape under pressure. Its four-inch diameter is more than sufficient for the 400-plus GPM discharge of a 3"x4" commercial gutter.
We've used these in both hot (Texas) and cold (Illinois) climates, and they've proven dependable and well-nigh indestructible. On the downside, they're not particularly pretty, and the tough plastic used means that it takes some effort to bend them. Overall, though, that 55-inch reach is invaluable for protecting your foundation.