This is a bit of an odd story as it's style doesn't really fit within the serious and scientific Season 18, the rather wacky idea of an intelligent cactus that can shape shift and wants to take over the universe sounds more like something from the mind of Douglas Adams and would have been more at home in the rather sillier Season 17. However, there is still much to enjoy here and I think it's quite an underrated story. The image of the Meglos Doctor is a very memorable one, seeing Tom Baker with cactus spines all over his face was quite a frightening image for me as a kid, and the make up is excellent. Tom does well in his dual role and gives a chilling performance as Meglos. The idea of the 'chronic hysterisis' is a clever one but after the third or fourth repetition of the same scene you start to wonder if it's just a way of padding out the story. The depiction of the Gaztaks is rather poor - they're clearly supposed to be a ruthless band of space mercenaries, but instead look like a rather clueless bunch of old dossers. Their costumes are clearly cobbled together from several older Dr Who stories. Some of the acting is a little wooden and the tone of the story seems to shift between humour and drama rather uncomfortably. The jungle set of Tigella is pretty good and some of the special effects are very good for their time (the giant screens on Zolfa Thura). The incidental music is also excellent.
The kidnapped earthling is very much a cliche, a bumbling English gent, but former Dr Who companion Jacqueline Hill is impressive as the high priestess, and there is a great sense of atmosphere in the sacrifice scenes, helped by excellent music.
An uneven story then, but nowhere near as bad as the reputation it seems to have.