Really successful, and unsurprising. By the end of this calendar year, it could very well hit SNES numbers. At the very latest, it will hit that benchmark by the end of the fiscal year with the release of Animal Crossing: New Horizons (which will be a system seller).
As it is right now, I'm confident the Switch will hit 3DS lifetime sales. Game Boy Advance numbers are relatively likely too. It's not very unlikely the Switch will surpass the Wii, but I don't see it happening. Despite the Switch being a hybrid (and a dedicated handheld model coming out), I really do think the PS5 and Xbox Scarlett will take some of its sales.
Lifetime Sales Predictions
PS4: 122 mil (was 100 million) Xbox One: 55 mil (was 50 mil) Switch: 79 million
3DS: 77 mil (was 73 million)
"Let go your earthly tether, enter the void, empty and become wind." - Guru Laghima