Arcade screens in the 1980s were roughly 800% larger than the PSP screen, so it can be hard to see enemy bullets in games like the vertical shooter Legendary Wings. On the bright side, many games whose monitors were taller than they were wide, such as vertical shoot-a-thons Last Duel and Varth, can be viewed in their original dimensions - you just change a setting in the options menu, rotate the PSP a quarter-turn and you're all set.
The controls don't always transfer gracefully to the PSP (especially for these same vertical games), though we'll give the developers credit for coming up with some creative solutions. For the side-scrolling, jet pack-and-big-gun blast-fest Forgotten Worlds, you can actually turn the PSP upside down, using the face buttons to move and pointing the analog stick to aim your weapon.
Another plus is that most of the games which had multiplayer modes in the arcade also have ad hoc multiplayer here, though true online and game sharing would have been even better.