Sequels to big movies are supposed to get gradually worse than the original. However in my opinion "Escape From The Planet Of The Apes" (the 3rd film) is the low point, and the 4th and 5th films both do a very good job of bringing the Planet Of The Apes saga round full-circle.
The whole timeline of Planet Of The Apes is circular, so in practice this means that the events in this film happen after films 3 and 4, but before films 1 and 2. It's a clever science fiction loop, but perhaps one that would work better as a novel than as the premise for a film, especially because you already know how events are going to unfold, more or less (although there are some interesting "can the history be re-written?" questions that could have been more emphasised).
Weak points in this film: the much lower budget is very evident, it feels once again more like a TV movie with unusually good costumes rather than a full-budget feature film. The actor Severn Darden is basically in the wrong film, playing a cringeworthy camp pantomime villain who doesn't really advance the plot in any way.
Overall it does stand up as a film, and if you've got this far with Planet Of The Apes, you might as well finish it off.