A huge improvement from Midway. Midway was a great game but it didn't have a high replayability. You could go through the campaign a few times and collect achievements or spend some time on the multiplayer but it felt pretty sparse in options. It may have been because I played Midway on the Xbox, but after about 70 or so total hours of playing it got fairly boring.
Along comes Pacific and ups the ante. You now have the option to play through the Japanese perspectives of the Pacific campaign, alongside the Americans, a skirmish mode with quite a few game types, and more multiplayer options (unfortunately multiplayer is dead for Midway due to the Gamespy servers closing however people are still able to play Pacific online).
The addition of new units, a special unit unlock system, and new nose art are also worthy mentions.
I will also note that you may encounter issues with Games for Windows Live. In order to play Pacific you need to have a Microsoft account. You may have problems activating your code, signing in or even starting the game.
Overall in the time I've played, its a great game. I'd highly recommend it to any history buff or anyone who wants to try something new that has a little dab of strategy alongside action. For the current price, I don't see why you wouldn't buy it. I would consider playing Midway first, it will help you get acquainted with the story if you aren't familiar with the Pacific campaign during World War 2, you'll enter Pacific hopefully looking for more, and its like 7 bucks so why not?