The story itself is fine, involving a renegade Time Lord's plan to destroy the universe and neatly appropriates the old legend of Hercules and Atlas for the crux of the tale. Pertwee and Troughton are both excellent in their different ways with neither hogging the limelight and bouncing off each other most entertainingly.
It was good to see the crusty Lethbridge-Stewart nonplussed at the appearance of the two docs as well as getting to finally enter the TARDIS. Katy Manning, as usual, gets to wear a mini-skirt and fawn over Pertwee, although she does at least come up with the idea for the feuding Doctors to pool their resources to effectively fight Omega. It might have been nice for say, Frazer Hines to make a showing too as Jamie, to assist Troughton and balance the ticket, but you can't have everything.
The only weakness is the usual laughably unscary monsters employed by Omega, which look like freshly-made jam puddings, but on the whole this deliberately nostalgic episode is an almost complete success and certainly one of the most memorable.