Another here learning Spanish on Duolingo, about 18 months in.
Confession: I have a degree in French and German, also Latin A level and did
O Level Italian in a year. I taught in 2ndary schools for many years and I do know something about language learning.
Truly, in my qualified opinion, Duolingo is a great introduction and the gamified system keeps one engaged.
But the truth is, in Spanish it is the verbs that a student really has to get a handle on. I have to use a grammar handbook as well, because Duolingo just presents new forms without explanation. And I'm someone who knows how Western European languages work.
Anyway, I'm looking forward to starting a local authority evening class this September.
A final thought: has anyone got experience of Babbel, a paid-for online language-learning system?