Thoughts on each side:
For the Sox, Kotchman's a pretty good 1B right now and he's in decent (not incredible but decent) physical shape. It seems they could teach him to play 1B/3B/LF at least, and maybe even a little RF for next year. He could be a mini-Youkilis for them in a bench role for next year. if they can do, and do do this, it's a good move for them, as LaRoche was worth nothing to the Sox at this point as a pinch-hitter/ backup or 3rd string 1B.
For the Braves, I actually really like this deal. LaRoche is a great 2nd half hitter, and he's a much better hitter than Kotchman when counting the 2nd half of the year only. It's an upgrade for the Braves for this year. And I wouldn't worry too much about 1B for next year, there are always plenty of fine 1B on the free agent market/trade market and guys like Huff, Delgado, etc. could be signed to 1-year deals. if worst comes to worst, Braves play Prado as everyday 1B next year (though that would be like a repeat of this year; good D and OBP with no power).