I think you've had your questions answered, so I will just list some games for you that I have played:
1. Resistance - Retribution: Great game once you get used to the button layout. The main character is cool.
2. FF7: Crisis Core: Great game, you can't explore as freely as FF's on main consoles, and the fights CAN be relegated down to mashing the X button (if you choose to do so). But the story is nice, and if you enjoyed FF7, you will enjoy the back story to Sephiroth, Cloud, and Zak. Can consume many hours.
3. God of War - Ghost of Sparta: Epic game. Big and bad-ass just like a GoW game should be! About as lengthy as its main console counter parts, maybe a couple hours less.
4. Crush: Puzzle game that lets you shift from 2D to 3D to solve each level. It's quite a lot of fun at first, and you do have to think to pass each stage. But I found that I grew tired of the game about half way through, and don't seem to have the incentive to complete it. Also, the music can be wacky and repetitive. Check it out first before taking the plunge.
5. Silent Hill - Shattered Memories: This game is ok, it's somewhat creepy at first, but once you understand the mechanic as to when you are in danger or not in danger, it loses it's edge. But it is an interesting take on the series, although admittedly, I am not the biggest Silent Hill fan. Actually, I am just not a fan. They're all just the same, with bland and tank-like gameplay/movement/fighting mechanics and empty environments. But it has its legion of fans and there is nothing wrong with that.
6. Loco Roco 2: This game is so effin cute! And it is fun to play as well! You rotate the screen with the shoulder buttons to move your blob around the place. Cool gameplay mechanics.
7. Hammerin' Hero: This game is ok. It has some stupid difficulty spikes, although I didn't give it much of a chance and kind of set it aside.
8. Half-Minute Hero: This is a fun, retro, and different take on RPGs. I haven't beat it but enjoyed what I have played so far. But I recommend you check out its style of gamplay before purchasing to see if it suits your taste.
I purchased Persona 3 Portable and MGS: Peace Walker back in the day when they had 50% discounts, but I haven't even played them so I cannot comment, but I do hear great things about them.
I also purchased Daxter for 4 bucks the other day and apparently that is one of the best platformers on the PSP.
I will be getting Pursuit Force and Killzone: Liberation from the Welcome Back package that Sony is offering. But I don't know if I will play those games anytime soon. Takes me months to complete a single game on my PS3 cause I can't dedicate as much time as I would like to gaming. Still playing Dead Space 2 ffs!
That is about as far as my PSP excursions go. I prefer digital downloads to UMDs, less clutter, can keep a back-up copy on my PS3, and I think I can transfer them to the NGP if I ever decide to buy one in the future. I have downloaded all the games onto my PS3 which doesn't take a lot of time as PSP games can range between a few hundred megabytes to about 1.5GBs, so nothing ridiculously large, although that will change once the NGP rears its head.And then use a USB cable to copy from PS3 to PSP. Simple process. Never had a problem.