I would say it is definitely worth £40 (but why wouldn't you get it for
£30?) if you are going to play the multiplayer even if it's only occasionally. The campaign itself is shorter than Halo 3's by a few hours but it has many great moments to match it and no missions at all that I'd consider bad or didn't enjoy.
The campaign certainly feels less epic than Halo 3's because it all takes place within a city with only one flashback offering any kind of wide open space. I can't really say one is significantly better than the other though.
Firefight is great but it takes a long time to start getting high scores and it is hard to get 4 friends at once to play so I don't see it as being as popular and constantly played as people had hoped. When you are playing though it is brilliant fun.
I'm not sure whether it was worth £30 for the ODST disc alone. While I am completely finished with the game on my own I still have many hours of co-op fun to be had. And that is not even counting the Halo 3 Multiplayer disc. The game is worth it for me even though I have all of the DLC maps because I will get so much use out of them. If you don't own the multiplayer maps however ODST is an absolute bargain and there is no reason not to buy it.