I've played thru and completed both Japanese and English versions of Peace Walker on PSP before. The experience was great, even though they were played under a single analog knob. This HD remake should bring Peace Walker to the proper recognition it deserve.
Visual and Aural
Peace Walker was originally for PSP, thus it can be seen that the models have less polygon count than what you'd expect from a PS3 / 360 games. However, even though with hardware limitation, it was an amazing looking PSP game. With this HD collection, the polygon count seems to remain unchanged, but the HD texture is good enough. The soundtrack on the other hand is amazing, in one of the battles, you're pitted against large number of enemies with a powerful ballad playing in the background.
Story and plot
Story-wise, Peace Walker takes place after MGS 3 Snake Eater and before the original MGS for PSOne, with Big Boss (Naked Snake) as the main protagonist. We get to see how Naked Snake evolve and finally accept the title Big Boss, and become the villain of the main series.
Peace Walker has less codec communication than its predecessors, as the matter of fact, you can't really call specific individual thru your radio. And it's actually a good thing, considering that MGS series always has long winded conversation. Cut scenes are done thru interactive motion comic, which means that there are QTEs in between, but still it's not the 9 hours long cutscene, and most of them are skippable and viewable later from separate menu. There's also tape recordings between Snake and others, which are accessible from the main menu, with conversations ranging from character's background to the wildlife in Costa Rica. They are optional, but to me it's one of the main draw of the game. The game has good pacing, and its own goofy moments, and even though the story is presented both thru radio conversation and comic cutscenes, they are still pretty cinematic and enjoyable.
Gameplay
Metal Gear is still Metal Gear. You're still awarded and expected to go thru missions stealthily. However considering that Peace Walker was originally developed for handheld, the whole game has been split into smaller missions unlike previous games. With the capability to go back to previous missions, and choose your load outs, one could spend large amount of time going back to each mission to achieve perfect ranking for it.
One of the new gameplay features is the addition of Fulton recovery system, which is essentially a skyhook. You could attach it to hostages, or to enemies which you have just knocked unconscious, and they will be deported back to your mother base. In the mother base you could assign the personnel to R&D, Intel, Medic, etc, and from there on, you could start researching on new weapons and gadgets. This RPG element further inflates the gameplay time to upwards of 60 - 100 hours.
Multiplayer is one of the new features, however up until this point I haven't had any chance to play with it.
Bottom line
MGS Peace Walker is an amazing game, in which you could spend countless hours playing in, regardless whether it's single or multiplayer mode. And since it's part of the HD collection with 2 other games, and costs less than $40, go and buy it.