Minecraft 64 x 64 HD Textures vs Default 16 x16 Texture Packs
Minecraft has always taken a great deal of ribbing about the state of its graphics, which are simple at best. Some modders have decided to fix the shameful graphic issue by adding 'HD' texture packs in which the tiles no longer display at 16 x 16 per tile, but instead sit at 64x 64 or even 128 x 128 if people are pushing the limits of what can be done.
If you just want to know how to install HD textures, I've written a guide here. So go forth. The rest of this article is going to be dedicated to discussing the relative merits of HD textures vs the default 16 x 16 texture packs.
HD Textures and General Prettiness
General Prettiness always had a hard time in the military, but he found some salvation in HD Minecraft texture packs. It's true, I suppose that HD texture packs are more visually engaging than the default textures. That won't necessarily mean that you like them more however. It's actually quite strange seeing the Minecraft world rendered in HD, and plenty of people go back to the default textures fairly quickly. This is purely a matter of personal preference and only you can decide what is you prefer.
HD Texture Packs and Minecraft Performance
People worry that performance will take a hit if they upgrade to an HD texture pack. I didn't personally notice that much of a decline in performance, if any at all. The vast amount of 'work' Minecraft forces your computer to do is in rendering the thousands of chunks as you move. The skin on top of those chunks is not as much of a factor as the chunks themselves.
Having said that however, if you are sailing close to the wind with Minecraft working at all on your pc, then you might want to shy away from HD texture packs.
HD Texture Packs Lack Flexibility
Default texture packs are easy to change according to your mood,desire and design aesthetic. At the moment I am using three texture packs, the default, Arabesque and ModernCraft in three separate worlds. My ModernCraft world is full of sleek developments, my Arabesque world is full of ruins (it's actually quite fun building ruins, you can build them from scratch in ruinous shape, or you can build them and then blow parts of them up and patch them with other materials for that 'just barely standing' look.
With the current state of affairs in HD texture packs, the HD texture pack overwrites the default and makes itself the only texture pack you can use. Your old (16 x 16) texture packs are still available on the 'Texture Packs and Mods' screen, but trying to use them will result in chaos and infamy.
EDIT: The truth of this statement depends on the HD texture pack being used. Misa's HD texture pack will not allow other low res textures to work, however some other HD texture packs, particularly those in the 32 x 32 range may be more forgiving.
Personally, I installed Misa's HD texture pack, wandered about agog in the world for 15 – 2 minutes and then went back to the default textures. I preferred the ability to easily use a variety of texture packs and the new high definition of the HD texture pack felt weird and scary to me. That is mostly a function of the way I like to Minecraft however. Your mileage will vary.