The Mark of the Rani is not very good, but not offensively so like The Twin Dilemma and Attack of the Cybermen. Kate O'Mara and Anthony Ainley are both good in it, at least. Some other points
1. What is the point of going back to Killingworth in the 19th century and having George Stephenson and then NOT ACTUALLY SEEING A WORKING STEAM ENGINE AT ANY POINT. Well? I can see an easy place to put it in the story, too: have the TARDIS be lifted out of the mine with one at the end.
2. I present this following transcript (I may have made up some of the stage directions here)
The Rani: [about the Master] What's he up to now? Probably something devious and over-complicated. He'd get dizzy if he tried to walk in a straight line.
[The Rani hears a knock, puts per hood back on in order to pose as an old woman who runs the bath-house of 19th century Killingworth, which she has gone back to with her time machine from her own planet that she rules, in order to extract a chemical from the brains of the working-men of this town which she is unable to synthesize despite being a genius chemist who can make land-mines that turn people into trees.]
3. I shall repeat that last part: land-mines that turn people into trees! And not just any type of tree, but semi-sentient trees.
4. They nearly give Peri something to do with her l33t botany skills, i.e. making a sleeping draught, but it turns out she doesn't know the plant life in Northumberland well enough, and it's only with the help of the amateur botany enthusiast Lord Ravensworth that she's able to identify the plant to use. This is particularly harsh as she has little to do in this story. And anyway, it turns out it's not needed anyway.
5. Apparently the Doctor can intimidate the Master [and the Rani, who knows of him by repute] into obeying his instructions by threatening them with a gun now. Contrast the much more credible "You never would, you coward" reaction when he's waving a gun around The End of Time. Now, to be sure, Six might very well do it, from what I've seen, but I don't think the Master watched the last episode, did he?