I loved Wii tennis when I first got my wii, but it always felt a bit unrealistic and easy. Other tennis games have tended to be a bit silly, so I tried table tennis games, but they are ridiculously hard.
Grand Slam Tennis is simply fantastic. You don't need motion plus, and in fact I am loving every minute of it without using motion plus.
The emotions you feel are so real, to the point I find myself telling the commentator (Pat Cash) to shut up.
My only gripe .... 'easy' setting is too easy and 'medium' is much more difficult, so I went from winning every match on easy setting without dropping a game to struggling to win a single game on medium setting.