Notoriously, the first episode of Invasion of the Dinosaurs exists only in black and white, while the other five are in colour (it would all have been in colour when shown in January/February 1974). Also notoriously, the actual dinosaurs themselves are absolutely terrible as special effects. There are no two ways about it: they are embarrassing puppets pasted onto their scenes by unconvincing CSO.
If you can ignore the awfulness of the dinosaurs, it's not such a bad story; like many Pertwee tales, it is a bit too long, but the two basic bits of plot - conspiracy at the highest levels of government to Take Over/Destroy England, and the people who think they are on a spaceship to colonise the nearest star - are both rather good and well enough worked out, with their motives a bit of a reprise of The Green Death but with the environmentalists now the bad guys. The cliff-hanger where Sarah is told that she's been in space for three months, and the scene where she proves she isn't by walking out of the airlock, are both real jewels.
The main plot twist involving the regular cast, however, is a slightly different matter. Captain Yates, the Brigadier's deputy since Terror of the Autons, turns out to be in league with the bad guys, yet can't quite bring himself to do the Doctor harm. The scene where we discover his betrayal is handled with no dramatic tension whatever, and his motivations are not really explored at all. The Brigadier and Benton get all the good lines, but there's interesting narrative tension among the villains as well.
If it hadn't been for the dinosaurs, this would probably be remembered as one of the great Pertwee stories despite the not-quite-connected plot. As it is, you just have to close your eyes when they are on-screen; but it's still way ahead of, say, The Mutants. (I wonder if an audio version of this, with linking narrative by Elisabeth Sladen or Nicholas Courtney, might work a bit better?)
When I expressed the view that none of the Sixth Doctor stories was any good at all, redfiona99, londonkds, loveandgarbage and wwhyte all recommended I try Revelation of the Daleks (with mild dissent from iainjcoleman and altariel). The Dynamic Doctor Who Rankings page currently has Revelation of the Daleks at a princely 51st out of 200 stories ranked, better by far than any other Sixth Doctor story - The Two Doctors is now at 100th place, and Vengeance on Varos at 102nd, while The Twin Dilemma has rather surprisingly been shifted from last place in the list by Daleks In Manhattan and Evolution of the Daleks, presumably by voters who have not actually seen The Twin Dilemma themselves.
I realised that I'm a bit out of phase with Davros' history - I have seen Genesis of the Daleks many times, and was then rather baffled by Remembrance of the Daleks (which is this month's pick on who_watchers), but have not seen either Destiny of the Daleks or Resurrection of the Daleks. I'm also not well up on the Sixth Doctor generally - I think I saw The Twin Dilemma, Vengeance on Varos and The Two Doctors when first broadcast, and this is the first of his stories I have watched since. (I must add the standard rider - "but the audio adventures are so much better" - because on the whole they are, especially the ones with companion Evelyn Smythe.)
I don't think it matters. This is almost a 1985 version of the more recent "Doctor-lite" episodes, with him and Peri not really there much, and Davros playing a role which is incomprehensible given his past and future as we know it. If you can overlook the huge plot flaw of Davros feeding the entire galaxy with the corpses of a few rich people, it's very well done, with various different factions of characters motivated for different reasons. In fact it's odd to consider that this was a time when, despite the generally lousy production values, really big names wanted to appear on Doctor Who - here we have Alexei Sayle, Eleanor Bron, Clive Swift (Hyacinth's husband in Keeping Up Appearances), and William Gaunt (who I can't remember seeing in anything else, but he is very good here). Shame about Jenny Tomasin but you can't have everything.
So, basically, I liked this - not a great story, but at least a good one, and definitely under-rated.