If you're going for a planted tank, half rounds are pretty challanging to plant with good view lines and I've seldomly seem them fit well in the flow pattern of a room for good access and viewing. Otherise, yeah, they look cool.
Hex tanks are interesting to look at, but not so great for looking INTO.
Peek at the corner of a regular tank and see how it distorts your view. Now multiple that. A big fish might be hard to see in one shot with smaller slanted panels. On the otherhand, if you're keeping huge schools of smaller fish, the distortions can make it look like you have more fish than you do.
The classic aquarium is a long rectangle for a very good reason--it gives the best surface area for the amount of water, is easily worked into/onto
furniture along walls, provides a long area for mounting equipment along the back and gives a large flat viewing surface.