It took a little over 2 months for my invite.  Signed up May 22, invite went through July 29, ordered sim July 29th, got welcome box/sim Aug 6.  So far so good, haven't noticed any difference from tmobile.  I usually use ~ 1 gb/month (I have good wifi access most of the day) so this could end up cheaper than $30/month...  we'll see.
ETA:  my number ported in like 2 minutes.  it was weirdly efficient.