Dude, you can get something like Midi Keys for free and a tiny host application for free & run all of the various free VST or Audio Units synths out there for free. At $25 you're half way to buying Garage Band and that includes all the other iApps.
Sorry, I give you threes across the board since I havent used it but a 1 for value. If you want to make a good VST or AU synth, that might be worth paying for but it needs to be good if I'm going to use it in place of something free.