This is easily one of the best Malayalam movies made. And one of the most iconic. The character of Mangalassery Neelakantan is as legendary a cult figure as it gets. The script was great, and so was the dialogue, both by Ranjith. No crass monologues as there would so often be a decade later. An example of the splendid dialogue (one example among many good ones) would be when Neelakantan talks about all the girls he'd romanced before as corpses, commending Bhanumati's character and calling her a real woman. Rarely in Indian cinema do we get a strong female lead (played so well by Revathi) going toe- to-toe with the male lead.
But the heart and soul of the movie was Mohanlal. Without him, there would be no Devasuram. The ease and spontaneity with which he essays that role is commendable. Such a role is easy to ruin with one- dimensional histrionics, but Mohanlal does not do so. Also, he always has that quality to bring a dash of vulnerability to even the most invincible of the characters he plays.
Even though this is one of my favourite movies I cannot give it a perfect 10 (I am tempted to do so). As great as it is, it could have potentially been even greater a movie, if it was handled with some more subtlety, like Irinjal rightly said. For example, the fight scenes were relevant to the plot, but a bit out of line with the realistic tone of the film, even with Mohanlal's larger-than-life character. The antagonist Shekharan could have been so much better than a bland, grimacing Napoleon. Imagine someone versatile like Siddique as Mundakkal Shekharan (sadly his versatility was exploited much later).