Benefit in entering Halls of The Dead?

Is there any particular benefit in entering and clearing (and looting) the Halls Of The Dead present in cities? Or is it just like clearing an ancient nord ruin? Also, what special items/loot (if any) is present?

Not really - it tends to be minor scaled loot. Occasionally a quest will take you them and each Hall contains a shrine to Arkay (increase health blessing).

There are some notable items within two of the Halls of The Dead:

Whiterun

Block Skill Book
Stone of Barenziah

Markarth

Restoration Skill Book

There's no real benefit; however I found it very useful in J'zargo's Experiment quest where I needed to find three undead fast without having to end up on a wild goose chase for some Draugr!