While I'm usually wary of giant corporate mergers like this, the fact that they are 3rd and 4th leads me to believe it would actually position them to better compete against Verizon and AT&T in a way that would be good for consumers.
If they don’t, Sprint tanks, and we still have three main market leaders, however two will remain significantly ahead of the third. Keeping in mind that the two that are ahead are now playing the monopoly game and buying into other industries (see AT&T and Time Warner or AT&T and DirecTV or Verizon and Yahoo, etc)
Approve or not approve, regulators are sometimes forced into a position of "picking winners" :-(
Another thing that needs thinking through is what happens to Sprint's spectrum if the company goes under. What kind of rules will be placed on that auction? Would ATT and Verizon be likely winners of such an auction?