I ran this module. My players loved it.
We had tons of role playing and very little combat.
Really, every adventure depends on the GM and the players. If your group seeks out hack and slash, they will find it regardless of what the module writer wrote. If your group seeks role playing, they will find it, again regardless of what the module writer wrote.
Note: I'm not saying that your group said, "We want hack and slash." I'm saying the group dynamic worked that way, regardless of whether it was intentional or unintentional.
Seriously, this is a module with tons of opportunities for alliances, stealth, and skulduggery. You can avoid almost every fight in the entire module if you choose and still succeed by foiling the plot of the final opponent.
On the other hand, your party of valiant heroes could decide to cleanse the Underworld of all evil beings.
Its all up to the players and the gm.
Also, use your search function. There are tons of threads on this module on these boards, the WOTC boards, and other websites across the internet. This is THE high level adventure for D&D 3e, besides the adventure path modules.