Immortality: This makes you invincible. No it doesn't last forever, there's a time limit, making this wish somewhat worthless. Best to use -after- you've boosted your PL with strength several times.
Strength: Adds 1 million to your power level. Always do this wish when you're in non-ascended form. Why? Example: Say you have 1,000,000 as your PL and the PL multiplier is 1.3 for transforming, so you transform and now have a PL of 1,300,000. Then you do the wish and your final PL is 2,300,000. Or, you get the balls, and do the wish while in regular form with your 1,000,000 PL. The wish makes your PL 2,000,000 and THEN you transform for 2,600,000 which is noticeably higher. If you're confused by this... no one can help you. Just take my word for it. If you gathered the Dragon Balls in SSJ mode to be quicker, remember to descend before performing the strength wish, then transform again afterwards. (Note that these numbers are not exact in-game values, just used to illustrate the truth of the matter.)
Friendship: I have no idea. Sounded retarded so I never tried it. Based on the name, however, I would assume that it turns all players into your friends who cannot harm you for a certain amount of time. Perhaps I'll go try it and edit this post, unless someone who knows posts before that.
Hope this is helpful. This is probably the third or fourth Dragon Ball question that I've answered. If it's not the subject of a sticky already, it probably should be... just a suggestion.