After an entire evening playing doubles with a friend here' some thoughts:
Standard mode - The super moves are fun but they're used way too often, them being used to simply cancel your super smash leaving you with no juice to defend is a major bummer. Missing a Super Smash block results in an instant KO and ends the match, if you like me suck at blocking them or if you're really good at using it, in both sitations the matches are shorter than the whole loading screen. I would prefer if it just removed a heart from your team or something, having a 1 minute match only to only have to go through the continue playing process again and loading takes the wind out of the sail.
Simple Mode - This is actually more enjoyable because you get really intense rallies and matches can take a bit longer but they're still way too short, there's no best of three, no way for a good comeback, no tennis scoring system that makes tennis, tennis. It doesn't feel good to loose or win this way. The fact there's no rematch option is also a bummer, just let us rematch on the same map, so it doesn't have to load one of those annoying gimmick courts that takes way too long. It's such a motivation killer, you can't really get in to it and gain momentum.
The base mechanics are solid but it lacks so many just basic essentials that makes tennis such an exciting sport. I want those 6 hour matches!