I stayed in Hamburg 4 nights before Wacken this year and then a week or so after Wacken. In my opinion it was awesome, it certainly beats anywhere i have been in the UK. There are many metal bars/pubs in St.Pauli and the Reeperbahn. Around the time of Wacken, it seems Hamburg is the mecca for all metal heads, so there seems to be alot going on wacken parties and such, all are very friendly and will offer to show you around the places that are must see attractions for any metalhead.
Some of the places i recommend checking out are:
the night light - great bar, great music, very friendly bar man speaks english, closes when the last person leaves.
3 monkeys - i think this is only open friday and saturdays, maybe more nights around the time of wacken, its worth checking out they have drink promos, really nice selection of drinks, a sofa area to chill, table fussball and a dj who plays requests.
steppenwolf - friendly bar staff, good music and band videos, sofa area to relax
there is a club on grosse freiheit just off the Reeperbahn which have different nights on in the week and at the weekend, i think they have a black metal night and while we were there some of the Germans we met took us to an ebm party which was quite fun.
der clochard - open pretty much all hours, good dukebox, balcony with seating, though we were told this is best when drunk!
Along the Reeperbahn itself is a heavy metal bar which sadly we didnt stay long enough to check it out, but it seemed to be full every time we walked past. There is another place called Molotow which we also didnt check out but i hear its a pretty cool place, and i think they have bands.
There are some metal shops along marktstrasse and near the hamburger dom and i definately recommend a visit to Remedy Records.
Hope this has been of some help