This is merely an effort to post here more often...
So I decided to switch my particle engine from the MS sample to using Mercury. So far so good. Had to rip out a ridiculous amount of code and basically completely change the way I was drawing everything, but in the end I think it'll be worth it.
Mercury's particle editor GUI is going to save me a lot of time editing effects without having to constantly edit and rebuild, look at the effect, and go back and revise it. The only complaint I have is that it's forced into using Immediate in SpriteBatch and that I have to deal with xml files for all the effects.
On the plus side, performance seems to be quite a bit better and it's a LOT better at allocating memory.