If you really want to use an adapter, get a XIM, don't bother with the FragFX or the other similar products. XIM gets better with each version from what I've read, but the original still works fine and is easy and cheap to make yourself (cheaper than a FragFX if you spend a few minutes shopping around for the parts). The XIM3 removes the need to use a computer with it I think, just plug the mouse and keyboard right in to the device, but I'm not sure what else goes in to making it work with the PS3, haven't looked in a while and didn't have much interest due to the cost. If you just want to enjoy a couple fps single player campaigns then it works great, and would be ok for some casual online multi. It's not a perfect 1:1 translation from what you're used to on a PC (new XIM versions should be better), but its good enough. It doesn't feel completely like a mouse on a PC everywhere on the screen, but is pretty damn close at the center. I know that some gamers who come from PC to consoles can adapt to the controller enough pretty quickly to get close to where they want with the crosshair, but then have trouble with the small aiming after that. The XIM will help a lot with that.
I have found that after spending enough time with the console controller in a single game that I get pretty good with it, though it takes a while again to get it back if I don't play for a while. I split my time fairly equally between my PC and consoles, and being much more comfortable with the M&KB its noticeable when I go back to the console. Some games it doesn't bother me as I can get to the point where the controls aren't a frustration and I can have fun, but with some games I just can't fine tune enough.
Unless you're looking to replace your PC with the console for your hardcore gaming, and you really want the M&KB, spend a few days at least with the controller, you'll probably get better with it than you expect. If not, and the controller is still driving you crazy, and you're going to play a bit more casual on the console, and older XIM will probably work fine. Again, stay away from the FragFX and other similar items, and don't drop the money on a XIM3 unless you're going to be using it a lot.