I'm less enthusiastic about this trade than most. I realize that most have given up on Lopez, but he's surely a better prospect than Johnson, who has gotten major minutes in his first two years and still can't shoot better than 40% from the field. A 22-year-old 7-footer is, almost by definition, a better prospect than a 25-year-old 6' 7" guy, especially if neither has yet shown anything to get excited about yet. I'm reminded of when the Suns gave DerMarr Johnson a look several years ago, after he had been a high draft pick (#6 by Atlanta in 2000) and hadn't gotten anywhere after a few years in the league.
So really it's Lopez for whatever the pick turns out to be, and Warrick for Johnson. Warrick for Johnson is a win for the Suns, probably, but we can say only "probably" because in fact Warrick has been the better player so far in their NBA careers -- and furthermore, if the Suns pick up Johnson's fourth-year option, which seems only logical, then the trade is a wash financially.
Is Lopez for a pick a good deal? Well it depends on the pick, of course, but generally one should not expect to get a better prospect than Lopez in the second half of the first round. We all know about the occasional success story, but that does not change the fact that the hit rate for late picks is quite low.
In four or five years, it's fairly likely that this trade will end up being that the Suns gave Lopez away for nothing. That could end up biting them.