Honestly, most 'shooters' or action games will not really be worth it on the Diamond or the Touch Pro. Controls are difficult at best and the lack of multi-touch or extra buttons does not help matters.
that being said, there are plenty of games that do work, and work well. Things from RPG's (phantasy Star, dragon warrior) to puzzle games like bomberman. If I wanted to do some heavy mobile gaming, I'd play with my wife's DS, but most emu's have a game or two that I cna play to kill a couple of hours without having to carry a DS everywhere I go.
Remember, you get what you pay for...and we paid for a device that does a bit of everything pretty good.
I've linked to the EMU's that I use, just in case it is your EMU is causing you or someone else some grief.
http://forum.ppcgeeks.com/showthread...mer+rom&page=5