Yeah, this pretty much exists on the strength of the leads' chemistry. As stories go it's entire inconsequential, and could be summed up as "the Doctor meets Martha's family; weird stuff happens around them".
That CGI scorpion is pretty atrocious. Also, I forget if it's ever explained how Lazarus has dormant scorpion genes that he could be (d)evolved into/out of/from. The whole thing is fairly nonsensical, even for a Doctor Who episode.
If one were to compare RTD's first and second eras, this story could be juxtaposed to "Lucky day" as a fairly humdrum monster used as backdrop for character buildup to seed a finale. Only that's sort of generous to "Lazarus experiment" while selling "Lucky Day" short.
Either way, this isn't an episode I rewatch unless it's for a full Martha-thon...
I mean, in Star Trek TNG s07, Barclay "devolved" into a spider or something, too, so I guess there's precedence?
Edit: "Genesis", blanked on the name of the episode...