Thieves Guild is one of the Gold levels that tends to get a lot of dislike.
Well, I can see why.
I recently played it again and have to say it´s not that bad. The idea behind it is interesting and adds a bit up to the story, because we don´t see much of Garrett´s activity before he is sent on the quest for the Eye.
On the other hand, this makes it also feel a bit out of place. It might also have been placed before Assassins.

Design-wise it´s a mixed bag. The sewers are of course not that inviting, not that I expected some fancy design here.
But then the walking through the tunnels can be somehow annoying.
If you get the knack of it, it isn´t that challenging either. Just a bit of a keyhunt, and that´s it. Even the missions exit is not really hard to reach.

There are tons of NPCs here. IIRC 69 should it be(not counting the spiders). So you might get a lot of KOs. Oh, and talking of the spider, why didn´t they change the faction of the big one(even named Kitty) to the thieves? Because they didn´t, sometimes they randomly start attacking the spider(and rarely succeed - but sometimes they do! Booh, poor spider!).

The soundtrack is a bit annoying. Some kind of humming sound. The casino soundtrack is fancy, but not heard that often. They should have used that more.

Overall, it´s not that bad that you want to skip it or play TDP instead. But then again, far from exciting.