As a character study of Odo 'The Alternate' works extremely well: the relationship between Odo and his Bajoran mentor, Dr Mora, is well written and powerfully performed by both Rene Auberjonois and James Sloyan (who has made several guest appearances in modern Trek over the years). Some of the scenes between them reached an emotional intensity not common on Trek.
Unfortunately, the 'alien on the loose' subplot is a little hokey in execution and almost seems to parody cheesy 50s B-movies. There are plenty of red herrings and some decent special effects (perhaps all the budget was spent on FX, explaining the somewhat unconvincing alien planet set). Still, in spite of the flaws, the emotional fireworks more than compensate for any weakness of the subplot.