I don't regret getting it on release day at all. It's been really good for me, despite Rosalina not being in. I've unlocked all characters, all clothes (not bought them yet) and got a star rating on most of the characters. I've also completed all the tournaments on Single and Double player.
Yes, I got it on release day. Votes: 2 (25%)
Yes, but not on release day. Votes: 1 (12.5%)
No, but I will in the future. Votes: 2 (25%)
No, I'm not getting Mario Tennis Open. Votes: 3 (37.5%)