I hate getting a film you loved as a child and watching it as an adult only to find it's completely lost it's magic over time. Having pushed all my videos to the back of a dark cupboard, I hadn't actually watched the film in years until I bought it last week on DVD.
Ferngully has it's problems, some of the songs were written with the days music heavily in the forefront so they haven't really stood the test of time. The overall message of taking care of the environment will seem heavy handed to an adult, although I remember feeling inspired by it as a child.
That aside I cannot vault the voice acting, Tim Curry, Robin Williams, Christian Slater, Samantha Mathis, they all deliver great performances. The animation is terrific and looks very much like Disney movies of the time. The stories hero's are compelling to watch, you really do care about what happens to them as it progresses to the climax, all this is woven together with touching little moments in the animation and a clever and funny set of supporting characters.
Will children today enjoy it? I'd say let them watch it, everyone has their own tastes and preferences and it's no different with children. They'll either like it or they won't.
I'm not sure if an adult who missed it as a child will enjoy it, again I think it depends on the person, but if this was a part of your childhood, it might be worth seeing it again I think it still definitely has a place in my heart.