I don't know about Crew but Toptal seems just like remote body shopping. Seems the freelancer gets locked in working for toptal. Not a real freelancing scheme like odesk and such.
Not really, you have full control over your availability. You can dial it up or down however you like. A lot of people start with hourly jobs on Toptal while keeping another gig or a day job and then switch to full time on Toptal when they're ready.
Disclaimer - I work for Toptal