Hello, I bought the HAF X and I agree that it is huge, it dwarfs my midsize Sony Vaio desktop. However, I do not need to move the case often and I wanted a superior, quiet case with USB 3.0 and would be tops at air cooling. The HAF X is all of that. The build quality is excellent. The alternate case to get would be the Silverstone Fortress, but it was nearly $100 more than the HAF X and I couldn't justify the price difference (I got the HAF X for $180 with no tax). Also the Silverstone is still USB 2.0 and I felt that it was dated for such an expensive case. My computer will be under my desk for my business and so size is somewhat irrelevant. And yes it just barely fits under my desk ha, ha.
It is really a subjective question. Practically, if you have to move the case often then a midsize case would be a better choice. In a way the bigger case is more handy because I can reach the I/O ports more easily and the optical drives. If you don't have to move your case often and are not always tinkering with your computer then the large size of the HAF X or other full-size towers doesn't really matter and has many benefits.
Also, the extra size of the case ensures that you have enough room for everything. When I researched midsize cases I constantly read warnings that a GPU wouldn't fit or a PSU. There are a lot of midsize case owners out there complaining that they couldn't get everything into their case. They also complain a lot that they don't have room for aftermarket coolers and cable management. Since I want to overclock, I need an aftermarket cooler and they are very large for the most part. Buying a large case like the HAF X eliminates all of these issues and saves a lot of guesswork. Also, should you want to expand your system by adding a second video card or PCI-E card, you can do it easily as long as you were smart enough to have bought the right mobo.
One other thing to consider is that many mid-size cases just don't cool as well as the HAF X and other larger cases (read the reviews). For example, I considered the CM 932 but I would have had to buy additional fans and size issues that I have mentioned would come into play. Buy the time I bought 2 or 3 additional fans, then I would be at the same price as the HAFX anyway and not have all of the benefits of the HAFX. Both the Antec DF and HAF X are rated in the top three for stock cooling - the Siverstone Fortress is the best air-cooled case in the reviews I read. Keep in mind that the Fortress is nearly as big as the HAF X.
Everything has tradeoffs and the HAFX is definitely huge. If I could have found a mid-size that met all of my requirements I probably would have bought it. The size you will be happy with will largely depend on how you use your computer.