I can't speak about Callison, but my friends working at ZGF seem to really enjoy it. It sounds like it has some of the issues inherent to a larger, more "corporate" firm, but beyond that, they've told me it's a supportive culture overall, especially for younger architects.

I will agree with what Gregory said - ZGF is a corporate firm but a good one. A lot depends on who you work for (IE: your boss) and which office you work in. I'm most familiar with the Seattle and Portland offices which are both good. They also have a solid business model and do excellent work - the work tends to the corporate, but the problems they solve are excellent & they do care deeply about green design. Younger architects do well there. 

Callison does mostly commercial (IE: retail) work (as I recall) and suffered greatly in the last economic downturn. ZGF does a lot of healthcare, office towers, public buildings, transportation and did OK during the economic downturn. 

I'm guessing you are in Seattle? If yes, I'd pick ZGF above Callison.

Love the ZGF office in Portland, very nice building, sweet staircase, and pretty cool folks, except for their receptionist...

ZGF>Callison, if you are a design focused professional.

Thanks everyone for sharing your insights! very helpful. ZGF seems like the way to go.

i worked at zgf portland for a little while

zgf LA office for design (portland office's best stuff is coming from about 6 people total so it's difficult to break into that group)

agree on the corporate culture -- you can get profiled/slotted there

agree on the friendly staff and smart/cool co-workers. if you're flexible you can get to travel and work out of different offices sometimes.

i don't know a thing about callison

Think of Callison as a slightly better version of RTKL. Indeed, now they are owned by the same congromerate (Arcadis)