I know it's an official card, but it doesn't sound like a very strong $5 to me at all. A handsize-neutral Warehouse? $5, really? I mean, a lot of the time Warehouse is good, but a lot of the time discarding 3 cards isn't even a big deal... I mean, maybe you have $10 out of the blue on a turn when you don't really want Provinces just yet, so you get this along with some other $5 card that you actually wanted...
Any time the composition of your hand matters, Forum is fantastic.
Think Crossroads, City Quarter, Menagerie, Throne Room, Crown...
Not discarding that third card is really great. It's a $5 that doesn't cost a Buy, and sometimes that makes it really nice.