On my first Ice wiz I did Sunken City at around level 15 because it seemed like I was supposed to do it before going to Krokotopia - and it was tough. With all my other wizards I have skipped it and Tomb of the Beguiler and then came back after I was a Grandmaster to complete it. I usually join a group of players that are under level 20 and help them cruise through it. A lot of new players think it has to be done and don't realize it is just a side quest; or even know how much harder it is than the rest of wizard city. There really isn't any compelling reason to complete it when you receive the quest so I advise people to wait until they are a higher level when they ask me.
Anymore i just let it sit in my quest log until someone else asks me to help them with it. Otherwise, the only reasons i see to do it are for the level 38 life spell quest (dryad) or if i need red mandrake for crafting. -eli and crew