I actually just stick to adsense and never tried others. But I heard from my friends that bidvertiser is a good alternative for adsense especially if your account on it is banned...
Good question, it is wise to use alternatives to Adsense, it is extremely lucrative though.
I currently only use Adsense, but I will start with alternatives. I have tried Bidvertiser on a Swedish website. I believe that it will be much better within the English word space. Affiliate is probably more lucrative, but it is not really the same thing as contextual ads (IMHO). Affiliate marketing, correctly used is very lucrative, but it requires more work than Adsense.