I still haven't played it lmao. Thief 1 & 2 are among my top favored games of all times the butthurt over the downgrade that was Thief 3 has never subsided for me. I'll concede however that compared to Thiaf it is probably a masterpiece.
You definitely should.
It is a 100% Thief game, in that it's heart is in right place, and also regarding the general atmosphere and direction, such as overall themes, art direction, Eric Brosius' music score, Stephen Russell voicing half of the characters, and all that jazz etc..
It greatly benefits from the fact that Thief was planned as a trilogy, first focusing on "Chaos" in form of Pagans, the second game going in an opposite direction with "Order" getting deviated with an advent of Mechanists, and finally back into the middle ground with the third game focusing on the Keepers/"balance", full circle (though Deadly Shadows is much more Dark Project than the Metal Age, with the suspense, undead and all). Those poor folks simply didn't have a chance to do it in DarkEngine - if Deadly Shadows was in that engine, you wouldn't even notice there was shit like the studio collapsing inbetween. In that regard it's even above New Vegas.
Virtually all of the criticism is due to how the console limitations hit the actual gameplay, the biggest offender of them all is of course those loading screens splitting levels in half. But, you can get a mods such as "Sneaky edition" or "Thief 3 Gold" that fixes that and more issues/conveniences. plus some John P texture packs or whatever that I never liked. Once you do disable that fucking loading screen, some of the levels actually get much more Thiefy, such as Wieldstrom museum or Widow Moira mansion (it's not like Ramirez's mansion was super huge either).
And then of course you have the Cradle, which for many people is the best Thief level of all time.
It's also such a highlight mainly because of how well it fits in the larger context of the game, when juxtaposed with all until that point, which you'll get when you see it (JarlFrank's contrarian rantings nonwithstanding). It definitely is as inspired as Thief 1's highlights, or Bank or Life of the Party from the 2, albeit for different reasons. Aside from that, I'd say the rest of the levels are alright, and consistent - nothing mindblowing like Life of the Party, although none of them as horrible as Thieves Guild or a redundant waste of time like Thief 2's level re-use. But once again, it definitely is worth playing just for the Shalebridge Cradle alone - and for the music, I'd say as well.
I definitely would play Deadly Shadows any day over those inane Thief Gold additions that ruined the perfect Dark Project for me, so much that I'm triggered every time I see that logo. Think of it as what New Vegas is to Fallout 1 and 2 and you'll be much more understanding of the game, even if not forgiving.