Hi there, everyone!
Hope I can offer some help as a native speaker of Spanish.
Not using the symbol ¿ to open a question is always a mistake. You'll always find it in official documents, textbooks, letters and so on. It is true, though, that due to the influence of English you'll see a lot of people writing things like "Qué tal?" in chats, whatsapp and the internet in general. It is a common use, but it's incorrect.
Some of you were also saying that it looks weird to have the symbol ¿, but it's necessary in Spanish, a language with extremely long sentences sometimes. I'll try to give you an example:
"Aunque tengas dinero y tengas muchísimas ganas de irte a París con tus amigos de la universidad, ¿no crees que deberías pensar en tu familia e irte con ellos de vacaciones?"
In English, for example, the question would start with "don't you think...", that is, a change in the order of words that indicates the beginning of a question. However, in Spanish we don't change the order of words, so you may not realize someone's asking a question until you reach the end of such a long sentence. If you're reading out loud it's weird, because if you don't see the ¿ or it's not written, you'll need to go back and read the sentence again to change the intonation.