OPL is as good as a usb/smb/hdd game loader can get on the ps2, however, if for use with usb, you will find most game barely playable with long loading time, stuttering in the fmv and, for some title, in-game stuttering because it only uses usb 1.1 speed. Also OPL doesn't allow you to play ps1 game.
You should also use free mcboot, this way you won't have to do the exploit every time you want to play, the noobie package include opl.
all you have to do is:
1. download the noobie package:
http://freemcboot.info/noobie.html
2. do the exploit -> ulaunch
3. run free mcboot installation package from your usb stick.
To play ps1 back-up, the only methods I heard of are disc swap exploit, meaning that you need an original ps1 game and your back-up will be on a cd (no usb loading)
