Game consoles controlled with a gamepad are not the best possible platform for first-person shooters. This is especially true for the PSP, as its controls are even more limited than a classic DualShock-style controller (the PSP has only one analog stick, which is much more limited in usability compared to the DualShock analog sticks, and it has only two shoulder buttons).
Thus creating a first-person shooter for the PSP so that it's even barely playable is a real challenge.
(Ok, technically speaking this is not a first-person shooter, but an over-the-shoulder third-person-perspective shooter. However, the gameplay is almost identical to a first-person shooter, so for all intents and purposes this is one.)
Syphon Filter: Dark Mirror succeeds in this surprisingly well. Naturally the gameplay simply cannot be as fluent as in a PC equipped with a mouse and keyboard, because such fluency in the PSP is simply a physical impossibility, but taking into account the limitations of the PSP controls this game is actually pretty playable.
A first-person shooter like this one has eight main movement controls: Four to move the character, and four for aiming. These already consume the majority of the main buttons of the PSP, and getting even slightly fluent in controlling the character requires some practice, but in the end it's actually surprisingly easy to learn.
Of course you can't hope to get aiming with just directional buttons which would be as fast and accurate as with a mouse, but an auto-aiming button makes this mostly unnecessary.
Sometimes playing becomes a bit awkward because of the limitations in the controls, but mostly this isn't a real problem.
One unfortunate consequence of the controller limitations is that they clearly had to adjust the difficulty of the game because of these limitations. In other words, the difficulty is such that if you were able to play it with a mouse and keyboard, it would be ridiculously easy. The low difficulty is compensated by the controller limitations, which make the game harder to play. In other words, big part of the difficulty of the game comes from the difficulty of controlling the character due to the controls, rather than anything related to the gameplay itself.
Of course it would be unfair to give a minus on this because it's absolutely not their fault: They had to deal with what the PSP has to offer, and adjust the playability according to that. They succeeded in overcoming these limitations pretty well, and this resulted in a game which is quite playable after a bit of practice.
The graphics are pretty decent, and eg. the different viewing modes (night vision, infrared, etc) work quite well and have a quite natural feeling to them.
The only part where the difficulty is really off-balance is in the final battle. That battle is just ridiculously hard due to the control system. Basically the final battle has infinitely-spawning enemies and limited ammo. You have to beat the main antagonist (which is extremely hard) before you run out of ammo, while trying not to die from all the constant enemy fire. Every time you kill a non-boss enemy, a new one spawns, ad infinitum. It's neverending.
If the number of enemies was finite then perhaps the final battle would be playable, but infinitely spawning enemies and limited ammo makes it almost impossible to beat. Once you run out of ammo you might as well just restart the level because it becomes hopeless. If it was a mouse+keyboard system, then it would be perfectly possible, but not with the PSP controls.
There might be some trick or tactic to be discovered in this final battle (as is often the case in these types of games), but I have not yet discovered it. All in all, it just seems fake difficulty, rather than a real challenge. It's a big downer.