There are only two dedicated bouldering walls, both run by the same company so with much the same setting style & atmosphere. Boulderwelt Ost is in the city centre; Boulderwelt West is a bit out of town so (slightly) less overcrowded on weekday evenings.
Of the routes-and-bouldering walls, the DAV one at Freimann (north, near the stadium) has by far the biggest & best bouldering area.
Thanks for all the replies. Ended up at Boulderwelt Ost for a wee climb last night. Good and relatively large wall with some interesting problems and a friendly atmosphere. Definitely busy though (at least in comparison to climbing walls in Edinburgh).