Trade works for both sides, particularly the Celtics. Detroit is motivated by the opportunity to fix the logjam in their front court, and everyone knows Monroe is the odd man out (and has said as much himself). Bass is clearly reserve with their line-up looking like Rondo-Stucky-KCP-Smith-Drummond.
Boston gets a lot better immediately with Jennings' shooting ability complimenting the more slashing-oriented Smart and Bradley. He also provides positional versatility, able to play either guard spot. I feel it would be best to have Jennings at the point and Smart at the 2 just for match-up purposes.
Monroe isn't necessarily the shot blocker we need, however he is 6'11" and is best suited to play Center for both offensive and defensive purposes, which he would in Boston. The C's starting 5 would look like Jennings-Smart-Green-Sullinger-Monroe, with Bradley, Olynyk, Young, and Pressey fueling the bench.
Overall I think both teams improve significantly, but more importantly the Celtics improve significantly. I think this makes us at least a .500 team with a lot of room to improve. In a few years, if we're able to lock up Jennings and Monroe long-term, improve the bench, and Smart/Young develop as planned, we'll be looking at a 50+ win team in my opinion.