I don't think it's really a question of whether it's still good or not - the original Acqua di Gio is exactly what it's always been, the thing is Giorgio Armani has released new versions and people get scared thinking they've changed the classic formula. The reality is the original version is still there, it's the same as it's always been. What's changed is that now you have options: reformulations, intense versions, gender variants... but if what you want is the one you've been using, that hasn't gone away.
The important thing is that you need to make sure you're buying the right version. If you go into a perfume shop or order it online, specify that you want the classic or original Acqua di Gio - don't mix it up with Acqua di Gio Profumo or the other ones they've released. Once you confirm that, it's exactly the same as always. Some people say classic products get reformulated a little bit over the years for regulatory reasons, but we're not talking about drastic changes that are going to disappoint you.
My advice: if it works for you, don't change it. I've found myself in the same situation with other things I've been using for ages, and the paranoia of "could it have changed?" isn't worth it. Unless you try the new bottle and really feel like it's different, stick with what you know you like. If at some point you genuinely notice something off, then look for similar alternatives, but I think you'll be fine.