I've bought several of these, and I've never experienced the issues some of the other reviewers have....

I think a key is that 1. i bought the right size spanx for me and 2. i have the correct size bra for me (i was actually fitted)

you really need both items to be the correct size.  if your bra is the wrong size (let's be honest - even oprah says 80% of women wear the wrong bra size but are in denial about it), the bra will fit too high on your back and of course there will be gaps.  you need a proper fitting bra, and pull it down in the back so it sits where its supposed to. I also put the spanx on first, then I put on my bra so it slightly overlaps the spanx.

I don't even use the straps it comes with to attach them together, i've never needed it.  I'm normally a size 16/18 and my bra size is 38F.  In these spanx, i drop one dress size.  it smooths out my stomach area and even helps with the dreaded awful back fat.  I do wish it did a lil more in the booty shaping area, but really, i can't complain.
I have also gone pee without taking it off - and i've never had any issues with "accidents".  it's all in how you position yourself when you sit down....just lean the right way and you'll be fine.
I think it's important to get the ones with the longer legs instead of the brief style bottoms. the brief style creates awful panty lines and for heavier girls like me, the longer leg style prevents nasty thigh rubbing/chafing. sorry to say, but its true.  It also important to get the one that goes all the way up to your bra line as the other ones create a muffin top.

I will continue to buy these as the ones i have wear out (the elastic gets slightly weaker with each wear and wash).