I would agree with Mr Cool - it depends on what you want to do, where and when.
A significant number of non-consulting buyers (though still relatively senior industry ppl) won't know who they are vs. a big 4 and so if your exit plan is ultimately industry you'll have the big 4 brand on your CV from now but the benefit is slightly tainted if they cant immediately identify who you are with today. This is my perspective from being on the other side of the recruitment process.
As a firm (and this is going back to the Structure Group days) I have heard from clients some very good things about their work and have had a number of very good and credible ex- colleagues of mine join them. Recently I have heard increasingly more negative things about their performance from clients - but given recent expansion this is perhaps not entirely surprising.
Would I join them? Personally no. I have held discussions in the past and despite the almost universal recognition of their very good culture, for me the lack of broader brand recognitiion, associated opprtunities and pay when you reach the v. senior levels isnt enough to make the switch.
I've tried to be as honest as possible but ultimately the answer is..... it depends!