I've been using the bitters from the exchange over the last few months, and wow - I not only appreciate how great all of them are, it's also had a big impact on how I make cocktails and has been a great kick in the ass to experiment more. Off the top of my head here's what I've been using most of them for (sorry if I've left any out, they're all great).
Turkob's Sumac Bitters - Works great with any brown liquors, kind of like Angostura with a deep spicier kick to it.
Hibiscus - I waited for warmer weather for this one, and have been enjoying a squirt of it it in beer - lighter Polish lagers and Weiss beers. Seemed to work with Prosecco as well (need to experiment with that one more).
My Thai Bitters - I didn't start using this much until it warmed up a bit, but it is great in Pimm's Cups and just used it this weekend in a Mint Julep which worked great - awesome. Probably would be good in a Mojito as well.
Fropone's Pear Bitters - I think this one is in a really nice place in between spicy and fruity, and can go in almost anything. We're doing a cross country drive this summer with limited room and I'm saving this one for our all purpose bitters on the trip.
Pimento Dram - This one's been really interesting, my favorite so far has been Bourbon, splash of Fernet Branca, juice from one lime and then about 10 drops of these bitters. Really nice! A good squirt of this can definitely stand in for some Allspice Dram in any recipe.
Ronnie's Oranillamon Bitters - I'm using this with great success in my regular cocktails - Boulavardiers, Remember the Maines, Old Fashioneds...another great bitter for the brown liquors.
Cardomom/Saffron - I really like this in Gin cocktails, and have been using it in Negronis. Donna strongly disagrees, but I'm sticking with it and will keep experimenting with this with warmer weather cocktails.
Pine Bitters - This one has been a big point of contention in our house. Donna has declared it to be bug repellent. I think a few drops of it make Vodka become a weird bastard cousin of Gin, and have been using it that way.
I'm sure I left some out, but overall I've been amazed working with all of them.