The content in TTK is astonishing - way more than the previous DLCs, and probably similar to vanilla launch Destiny. Possibly more, in fact. I've paid somewhere between £100-130 for Destiny & all DLC so far, but I don't feel cheated. I haven't played a game this much since Gears of War and then Halo 3.
The other point to make is that the polish on the game is now something else: the story is solid & coherent, the dialogue is entertaining (& much better use is made of the cracking voice cast), and the manner in which activities and quest lines progress is much, much more satisfying and all radiates out from the core narrative. Previous lore has been referenced to good effect to build the world and enhance the story. It all makes the first year feel like an extended beta to some extent.
I still think the light system needs a bit of work (contrary to previous claims, Bungie hasn't prioritised traditional XP over light; it still matters enormously at the level cap).
I do disagree with locking players out of old content, but it's not a 'pure' lockout in my view. Equivalent activities exist for all but VIP patrol missions: strikes are still available, . Yes, Nightfalls have technically gone unless you have TTK, but this is an end-game activity, and the end of the game is now Oryx, not Crota. Also, year one exotics can still be bought from Xur as legacy engrams, and PVP gametypes still exist on legacy playlists for those without DLC. Patrol remains available on vanilla Destiny (except VIP missions); the story with Nolanbot can be revisited.
I was honestly sceptical about the £40 price tag for TTK, but it's been worth it for me. Genuinely the best DLC yet, by far.