I'm using ZS right now as well, and I have been for a while now. I love it, but I've been wanting something new for a few weeks now too.
I tried Liberty GB, and it seemed buggy. I think when I tried, it was .4 or .5. But .8 is out now, so they may have some of those bugs worked out now. I LOVED Liberty 2.0.1 before the GB got leaked, so I'm hoping Liberty GB gets more stable quick.
From what I last saw, CM7 for the D2 was still buggy as well. Camera, GPS and in-call mic were not working. That was Friday, so someone may have made some progress on that over the weekend, I'm not sure.
This is another one that I can't wait to try, but those 3 bugs are big issues to me so they need to be fixed first. The D2 is my first Android phone, so I've never had the chance to use CM before, and I'm very excited to check out CM7.
EDIT: Oh yeah, and the alt functions (the blue keys) on the physical keyboard weren't working, but someone had a .zip file that could be flashed though recovery to fix that.
Haven't tried APEX, but when I used the Froyo version it messed up my phone pretty bad, so I ended up going back to liberty. (Again, before there were any GB ROMs.) I flashed it twice, and both times it was buggy to the point of not being able to use my phone, so that kinda soiled my opinion of APEX.
That's my take on them all, hope it helps!